Let us first dissect the sentence.
In nests across North America, - this phrase modifies the noun host mother.
the host mother tries to identify their own eggs and weed out the fakes, - SV pair mother tries. The possessive noun Their is not correct in this sentence since were are referring to only one mother
but the brown-headed cowbird
– a brood parasite that sneaks its eggs into other birds’ nests –
produces eggs that look very similar to those of the host, making that task surprisingly difficult.
SV pair - Cowbird produces
Making the task modifies the complete clause before comma.
(A) the host mother tries to identify their own eggs and weed out the fakes, but the brown-headed cowbird – a brood parasite that sneaks its eggs into other birds’ nests – produces eggs that look very similar to those of the host, making that task surprisingly difficult
The possessive noun Their is not correct in this sentence since were are referring to only one mother
(B) the host mother tries to identify its own eggs and weed out the fakes, but the brown-headed cowbird – a brood parasite that sneaks its eggs into other birds’ nests – produces eggs that look very similar to that of the host, making it surprisingly difficult
America Very similar to that of is incorrect. Very similar to those of is needed to make comparisions.
(C) host mothers try to identify their own eggs and weed out the fakes, but the brown-headed cowbird – a brood parasite that sneaks its eggs into other birds’ nests – produces eggs that look very similar to the host’s, making that task surprisingly difficult
This is the correct choice.
(D) host mothers try to identify their own eggs and weed out the fakes, but the brown-headed cowbird – a brood parasite that sneaks its eggs into other birds’ nests – produces eggs that look very similar to that of the host’s, making it surprisingly difficult
America Very similar to that of is incorrect. Very similar to those of is needed to make comparisions.
(E) host mothers try to identify its own eggs and weed out the fakes, but the brown-headed cowbird – a brood parasite that sneaks its eggs into other birds’ nests – produces eggs that look very similar to those of the host’s, making that task surprisingly difficult
Identify its own eggs is incorrect, sneaks its eggs is incorrect. The antecedent to the pronoun its is ambiguous. those of the host’s – is incorrect
Answer is C.