Before we get to the point of when to use and when to omit "that" is a sentence, first let's discuss HOW "that" is used in a sentence.
"That" is used in two roles in a sentence:
a. as a connector
or to be more "grammatical" as a subordinating conjunction
-> They connect a Dependent Clause (DC) to an Independent Clause (IC).
b. as a relative pronoun modifier
that gives additional information about the preceding noun entity in a form of a clause. The thing to note here is that they either may be the Subject of the DC they start or may start a DC.
Let's now take the example of each type:
1. We all know that
the Sun rises in the east. -> In this sentence, "that" acts as a connector and joins the IC (We all know) with the DC. |
2. I bought the umbrella that
was blue in color. -> Here "that" is Relative Pronoun Modifier as it provides more information about"the umbrella", the preceding Noun Entity. Here, "that" acts as the Subject of the Dc that it starts.
3. The pizza that
my brother yesterday was extremely delicious. -> Here again, "that" is Relative Pronoun Modifier as it provides more information about"The pizza", the preceding Noun Entity. Here, "that" DOES NOT act as the Subject of the DC that it starts.
Now "that" can be omitted from a sentence when it DOES NOT act as the Subject of the DC that is starts. This omission most certainly includes the usage of "that" as a connector because in this role, "that" NEVER acts as the Subject of the DC. This omission also includes the usage of "that" as a Relative Pronoun Modifier when it DOES NOT act as the Subject of the DC it starts.
So sentences 1 & 3. can actually be rewritten as:
1. We all know the Sun rises in the east.
3. The pizza my brother yesterday was extremely delicious.
However, we CANNOT omit "that" from sentence 2. because there "that" works as the Subject.
Now let's come to the sentence that you have cited: Of all the corporate "green" policies, this is the one (that)
I find most disingenuous.
So here "that" is omitted after "one". Here it acts as a Relative Pronoun Modifier because it provides more information about the preceding noun "the one" that stands for one of the green policies. Now "that" can be omitted here because it DOES NOT act as the DC that it starts. The Subject of that DC is "I".
For more on this topic and analyses of some official examples, please read the following article:learn-how-that-can-help-you-demystify-a-long-sentence-138358.html
Hope this helps.