Is answer E because 'is' correctly used with it
Using 'and' makes it unambigous

The starling, a non-native bird species introduced to the United States to control insects, are now the country’s most common bird.


A. The starling, a non-native bird species introduced to the United States to control insects, are now the country’s most common bird. Incorrect

SV error - The starling singular, are plural

B. Starlings are a species of non-native bird introduced to the United States to control insects and now the country’s most common bird. Incorrect

Starlings plural, common bird singular

C. The starling is a non-native bird species introduced to the United States to control insects and is now the country’s most common birds. Incorrect

The starling singular, common birds plural

D. Starlings are a non-native bird species introduced to the United States to control insects and now the country’s most common birds. Incorrect

Starlings plural, a non-native bird species indicate Starlings - which is wrong

E. The starling, a non-native bird species introduced to the United States to control insects, is now the country’s most common bird. Correct

The starling, a non-native bird species introduced to the United States to control insects, are now the country’s most common bird.


A. The starling, a non-native bird species introduced to the United States to control insects, are now the country’s most common bird.
The starling is singular and are is plural. S-V Error

B. Starlings are a species of non-native bird introduced to the United States to control insects and now the country’s most common bird.
and distorts intended meaning. use of and conveys both the activities are dependant with each other that is not correct.

C. The starling is a non-native bird species introduced to the United States to control insects and is now the country’s most common birds.
The starling is is singular and the country’s most common birds is plural.
and distorts intended meaning. use of and conveys both the activities are dependant with each other that is not correct.

D. Starlings are a non-native bird species introduced to the United States to control insects and now the country’s most common birds.
and distorts intended meaning. use of and conveys both the activities are dependant with each other that is not correct.

E. The starling, a non-native bird species introduced to the United States to control insects, is now the country’s most common bird.
No Remarks
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The starling, a non-native bird species introduced to the United States to control insects, are now the country’s most common bird.

A. The starling, a non-native bird species introduced to the United States to control insects, are now the country’s most common bird. Incorrect use of plural verb "are" to refer to singular noun "The starling". Eliminate.

B. Starlings are a species of non-native bird introduced to the United States to control insects and now the country’s most common bird. Singular "bird" incorrectly used to refer to plural "Starlings". Eliminate.

C. The starling is a non-native bird species introduced to the United States to control insects and is now the country’s most common birds. Plural "birds" incorrectly used to refer to singular "starling". Eliminate.

D. Starlings are a non-native bird species introduced to the United States to control insects and now the country’s most common birds. Incorrect usage of plural "birds" along with superlative form "most common". There can only be one "most common" bird. Starlings could be "among the most common birds". Eliminate.

E. The starling, a non-native bird species introduced to the United States to control insects, is now the country’s most common bird. Correct answer - above errors are corrected and no new errors are introduced.
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VERITAS PREP OFFICIAL SOLUTION:

The easiest decision point to deal with on this problem is that of subject-verb agreement. The only difference between (A) and (E) is that (A) uses "are" while (E) uses "is," a telltale sign that you need to determine what the subject of the sentence is. For each choice that subject is the singular "the starling," so the plural verb "are" is incorrect and you can eliminate (A).

Choice (C) also commits a singular-vs.-plural error, leading with "the starling" but calling it "the nation's most common birds," plural. (C) is therefore incorrect.

Choices (B) and (D) correct the subject-verb agreement error by matching the plural "starlings" with the plural "are." But note that each leaves itself open to a logical meaning error. if you slash-and-burn away some excess description, the core of those sentences is:

Starlings are a species introduced to eliminate insects and now the nation's most common bird(s).

While at first glance that might seem logical (Starlings are X and Y), note the placement of "and" could lead people to read the sentence as "Starlings were introduced to eliminate insects and now birds" - an illogical meaning as you cannot change the reason that starlings were introduced ("and now birds" suggests that change). This lack of clarity and ultimately illogical meaning means that (B) and (D) are incorrect, leaving (E) as the correct answer.
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