For a variety of reasons -- financial worries, dementia, loneliness

Rules of using Colons (:) ->
1. What follows the colon should provide further explanation for what comes before it.
2. We should be able to insert "namely" in place of colons and the sentence should make perfect sense
3. We should be able to insert "that is" in place of colons and the sentence should make perfect sense
4. Whatever comes before colon should stand as a complete independent sentence.

A & B) If we replace the colon with "namely" or "that is", the sentence doesn't make any sense. The mass market fraud is not deceptive pitches, but the act of receiving deceptive pitches via various mediums. Hence, A & B is out.
C) No antecedent of "they"
D) Correct; the clause "deceptive pitches that....." act as Noun + Noun Modifier in which Noun => Deceptive pitches
E) "Which" incorrectly modifies the mass market fraud, again implying that the deceptive pitches are fraud.

A) as mass market fraud: deceptive pitches that arrive by telephone, in the mail, and the Internet --- The clause has no prepositional parallelism. That arrive – by telephone, in the mail and ( no preposition) the internet
B)to be mass market fraud: deceptive pitches that arrive by telephone, the mail, and by the Internet ---- 'known to be' is unidiomatic, 'known as' is the correct idiom
C)to be mass market fraud; they are deceptive pitches that arrive by telephone, in the mail, and the Internet—same as in B
D)as mass market fraud, deceptive pitches that arrive by telephone, mail, and the Internet – The correct choice; the lack of prepositional parallelism has been amended by dropping the preposition for the second and the third item
E)as mass market fraud, which are deceptive pitches that arrive by telephone, mail, and through the Internet – The second item namely ‘mail’ is not having a preposition while the other two have.
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Option D it is .. well explained above !
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Sir
In your explanation of "as mass market fraud, deceptive pitches that arrive by telephone, mail, and the Internet " The definite article is placed only before one item and not the other items. Does this have any relevance in general because i have seen in some questions where the placement of"the" is a decision point.
daagh

Raksat , can you remember the issue with the presence or absence of "the"?

I recall only one such question at the moment, which I read today.
In that case, "the" was required in order to specify a particular kind of snail.
"The" needed to precede "freshwater snails" because the sentence
identified a particular kind of snail (freshwater snails that host a disease-bearing parasite).

Do the questions you recall involve choosing between
a general noun and a specific noun? For example:

Someone should call a doctor! (Any doctor)
Someone should call the doctor! (A specific doctor)

The sentences in this problem involve prepositional parallelism
by telephone, in the mail, and [----]the Internet
by telephone, [____] the mail, and by the Internet
by telephone, in the mail, and [-----] the Internet
by telephone, mail, and the Internet
by telephone, mail, and through the Internet

The reference to "the Internet" is correct.

1) the Internet, whether whether capitalized or not requires THE

2) That said, the preposition should probably be "THROUGH"
the Internet, not "BY."

Regardless of which preposition shows up, "Internet" requires
the definite article "the."

"arrive . . . through Internet" is not correct
"arrive by . . . Internet" is not correct

CORRECT: I found the recipe on the Internet.
WRONG: I found the recipe on Internet.

CORRECT: He downloaded the book from the Internet.
WRONG: He downloaded the book from Internet.

CORRECT: She found the job through the Internet.
WRONG: She found the job through Internet.

CORRECT: We communicated with one another over the Internet by using Hangouts.
WRONG: We communicated with one another over Internet by using Hangouts.

In the list of objects (phone, mail, Internet), "Internet" requires "the"
and the other two do not. That fact by itself does not create a lack of parallelism.

If I wanted to describe change over time, for example,
I might say, "The Internet has revolutionized methods of communication,
access to information, and the culture of this nation."

The first two nouns do not require an article. The third, in this context, does.
That sentence has no parallelism errors.

As long as you are able to identify the distinction between A and THE,
and to identify very occasional instances in which specificity requires THE,
I would not worry about the issue.

Even in the OG question to which I alluded (snails),
other errors in the options made the options incorrect.

If you can find OG questions in which the correct
answer hinges on the presence or absence of an article,
definite or indefinite, and you have doubts or questions,
find where the OG questions are posted on this site and
express your doubt.

We will be happy to help.

I hope this answer helps.
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I have a query about option D
There are 2 lists:
1st list is:
...Vulnerable to 1) mass market fraud 2) deceptive pitches
2nd list is:
deceptive pitches that arrive by --> 1)telephone, 2) mail, 3) the Internet
My query is:
I feel that there should be "and" between the 2 items of the first list.
D) as mass market fraud, and deceptive pitches that arrive by telephone, mail, and the Internet
Can somebody explain why there is no "and" in the answer choice D?
daagh, Bunuel

They're using deceptive pitches to describe mass market fraud.

... what is known as mass market fraud, deceptive pitches that arrive by telephone, mail, and the Internet.

According to the sentence, mass market fraud is deceptive pitches that arrive by telephone, mail, and the Internet.
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moul

Are there really two lists? I see only one item, that is, mass market fraud. And this mass market fraud, 'a collective noun' comprises only one item, that is, deceptive pitches that arrive via three channels. The core is that the deceptive pitches themselves are the mass-market fraud and not one different from each other.

To make things a little more explicit, let's analyze each choice deeply. A and B contain a colon and what follows the colon describes the mass-market fraud.
In C, one cannot even know what the pronoun 'they' effectively stands for, whether reasons or worries or senior citizens.

In D and E, a comma separates mass-market fraud from the deceptive pitches, indicating that the noun phrase after the comma is just a modifier defining the mass-market fraud.

The only issue seems to be that those three channels are not propositionally parallel in all the choices except in D.

Therefore D
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