To solve this question, the user needs to know subject-verb agreement. In this sentence, the subject is "much of the machinery" which is a singular noun phrase, so the correct verb form should be singular as well.

Now, let's go through each option and explain why it is right or wrong:

A. are: This option is incorrect because "are" is a plural verb form, which does not agree with the singular subject "much of the machinery."

B. None of the above: This option is incorrect because there is a correct answer, which is option D.

C. Both a and b: This option is incorrect because there is only one correct answer, which is option D.

D. is: This option is correct. Since "much of the machinery" is a singular noun phrase, the correct verb form to use is the singular "is." Therefore, the correct sentence is "Much of the machinery on these farms is unusable."

The Answer is: D