The webbed feet of the Alabio duck has neither nerves nor blood vessels, which renders the duck incapable of feeling the cold on its feet.
(A) has neither nerves nor blood vessels, which renders the duck incapable of feeling the cold on its feet.
The original sentence has a subject/verb agreement error. Multiple feet "have", they don't "has." Also, the usage of the noun-modifier "which" is not ideal, as the reason the duck can't feel the cold is the absence of blood vessels, not the blood vessels themselves.
(B) has neither nerves nor blood vessels, which renders them incapable of feeling the cold on their feet.
The original sentence has a subject/verb agreement error. Multiple feet have, they don't has. Also, the usage of the noun-modifier "which" is not ideal, as the reason the duck can't feel the cold is the absence of blood vessels, not the blood vessels themselves. The answer choice also introduces a pronoun issue, where it is not entirely clear whether "which" refers to the duck or to something else.
(C) have neither nerves nor blood vessels, rendering them incapable of feeling the cold on their feet.
While the use of "have" solves the subject/verb agreement error, this answer choice introduces a pronoun issue. The usage of "them" makes it unclear whether the feet or the duck is incapable of feeling the cold.
(D) have neither nerves nor blood vessels, rendering the duck incapable of feeling the cold on its feet.
This answer choice solves the subject/verb agreement issue, noun modifier error, and does not introduce a pronoun issue.
(E) have neither nerves nor blood vessels, rendering them incapable of feeling the cold on its feet.
While the use of "have" solves the subject/verb agreement error and the noun modifier issue is addressed, this answer choice introduces a pronoun issue. The usage of "them" makes it unclear whether the feet or the duck is incapable of feeling the cold.
Therefore, the correct answer choice appears to be D.