All of the university’s faculty is appalled by the decision to hire as university president the senator censured for influence peddling, but some are willing to extend for him a civil welcome.

A. are willing to extend for

B. is willing to extend to

C. are willing to extend to

D. is willing for extending to

E. are willing for extending for
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Compilation of Blogs by Mike Mcgarry - Magoosh

C is the answer. 'Some' is Plural. Please update that.

Official Explanation by Mikemcgarry on his blog :
Split #1: SV Agreement. The subject of the second clause, “some”, refers back to the “the university’s faculty”. This is a collective noun. While there are many faculty members, the faculty is singular, and “all of the faculty” or “some of the faculty” must be singular as well. Notice the singular verb in the first half of the sentence. The second must be singular as well. Choices (A) & (C) & (E) are incorrect.

Split #2a: idiom with “willing”. The adjective “willing” take an infinitive: willing to do X. The structure “willing for doing X” is never correct. Choices (D) & (E) make this mistake.

Split #2b: idiom with “extend”. When we give someone a welcome, we say that we extend the welcome to him. Use of “for” in this context is never correct. Choices (A) & (E) make this mistake.

For all these reasons, (B) is the only possible answer.
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Compilation of Blogs by Mike Mcgarry - Magoosh

SANAM - Some, Any, None, All, More/Most can be singular or plural depending on the context of the sentence.
In this case, some refers back to the singular 'faculty'.

Official Explanation

Split #1: SV Agreement. There is a subtle SVA issue here. If we are talking about the group of individuals, when do we use the singular verb for the group and when do we use the plural verb for the separate individuals? If whole group, more or less, is acting as a whole in some way, we emphasize the group and use the singular verb: for example, when we discuss what "most" are doing in the first half of the sentence, we use a singular verb. By contrast, if we are discussing how some members of the group are diverging from others and acting in individual ways, then we emphasize the individuals and use the plural verb. For the verb in the underlined section, the plural verb would be preferable. Choices (C) & (D) make a less than optimal choice in choosing the singular verb.

Split #2a: idiom with “willing”. The adjective “willing” take an infinitive: willing to do X. The structure “willing for doing X” is never correct. Choices (D) & (E) make this mistake.

Split #2b: idiom with “extend”. When we give someone a welcome, we say that we extend the welcome to him. Use of “for” in this context is never correct. Choices (A) & (C) & (E) make this mistake.

For all these reasons, (B) is the only possible answer.

FAQ: Can you talk a little more about when to use "is" and "are" for these collective nouns? I'm confused.

Sure! Dealing with agreement with collective nouns can be subtle, because you have to tap into the MEANING of the sentence to make the right call. Specifically,

if the sentence stresses the uniformity or 'togetherness' of the group, then use the singular;
if the sentence stresses individual actions (perhaps grouped together, but still regarded as clearly individual), then go with the plural.
If you are completely in doubt, err on the side of going for the singular option. This is not foolproof, but we find that most often—especially on the GMAT—the bias is toward the singular.
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Thanks!
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Simple strategy:
“Once you’ve eliminated the impossible, whatever remains, however improbable, must be the truth.”

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My Notes:
Reading comprehension | Critical Reasoning | Absolute Phrases | Subjunctive Mood