Thank you celidh,i can see why d is wrong but i have following doubt.the patchwork of green fields that surround / surrounds the San Joaquin Valley town bustles with farm workers, many of themin the area just for the season.i choose surrounds because of one of + that always singular.AM i right/
I think you're applying a rule in a context that doesn't fit.
What I think you're referencing is the fact that "the number of" or "a number of" are structures sometimes referred to as SANAM pronouns, and they behave differently than their structure would suggest.
Usually, when we have a collective noun or indefinite pronoun followed by a prepositional phrase, it is singular. For example, all of the following would be singular:
The group of students is advanced.
The board of directors is meeting tomorrow.
Each of the consultants has an MBA.
Either of the answers is correct.
However, there are exceptions that we call the SANAM pronouns: Some, Any, None, All, More/Most (there are others that behave this way, but these are the most common, hence the name). These can be either singular or plural, depending on the object of the preposition. Both of the following are correct:
Most of the money was stolen.
Most of the items were stolen.
Other expressions such as "majority of," "the bulk of," etc, will behave in this way. "A number of" would also theoretically behave this way, except that it would be strange to say "a number of [singular noun]," so we almost always use it with a plural.
But none of that even applies in this problem. In D: "... to reduce the number of
guests that
ends..." The issue was not whether "number of ___" had taken the correct verb, because that expression was not the subject, it was the object of "to reduce." The verb in question was part of a modifying dependent clause. When we use "that" to create a modifying clause, it will be singular or plural depending on whether the noun it's modifying is singular or plural. If it's the GUESTS who are doing the EXPRESSING, then "guests" is the subject of that verb.
The example that you gave is similar. Imagine diagramming it like this:
the
patchwork ...
bustles with farm workers
...................... \ of green
fields
............................................ \
that surround the San Joaquin Valley town
"that surround" is a dependent clause modifying "fields," so it's plural. The fact that it's preceded by "patchwork of" is irrelevant - that subject has its own verb, "bustles."
Does that help?