Cows have been sacred in India for centuries, and

The subject of this sentence is the plural noun "cows." Hence, any pronoun or other word that refers to "cows" must be in the plural form. This means that, in the underlined portion, the singular "it" and "creature" are incorrect. Eliminate (A).

A quick vertical scan reveals a 2-2-1 split at the beginning of the choices, with (A) and (B) using the singular "it," (C) and (D) using the plural "they," and (E) getting rid of the pronoun altogether. With regard to "creature," there is a 3-2 split, with (A), (D), and (E) using the singular "creature" and (B) and (C) using the plural "creatures." Combining the two splits together, it appears that only (C) uses a proper plural pronoun and noun. Expect to choose (C), but examine the other choices to be sure nothing was overlooked.

As noted, (A) uses incorrect singular forms of "it" and "creature" to refer to the plural "cows."

(B) uses the incorrect singular "it" to refer to the plural "cows."

(D) uses the correct plural "they," but it refers to "cows" as "a holy creature." Since "cows" is plural, what is needed is the plural holy creatures.

(E) not only uses the singular "a holy creature" to refer to the plural "cows" but also creates a fragment. Something more is expected at the end of the sentence: "and being still regarded today as a holy creature by Hindus, most of whom are strict vegetarians ..." what? Eliminate (E).

(C) changes the singular "it" to the correct plural "they," and it changes the singular "a holy creature" to the correct plural "holy creatures." (C) introduces no new errors and is therefore correct.

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Cows have been sacred in India for centuries, and it is still regarded today as a holy creature by Hindus, most of whom are strict vegetarians.

A. it is still regarded today as a holy creature
Cows-plural, it - singular

B. it is still regarded today as holy creatures
Cows-plural, it - singular

C. they are still regarded today as holy creatures
CORRECT. Cows-plural, they - plural. "Regarded as" is the correct idiom.

D. they are still regarded today as being a holy creature
"Regarded as" is the correct idiom.

E. being still regarded today as a holy creature
- This sentence is a fragment