Which verb agrees in number with its subject?
the saucepan or the stockpot wide enough for all the potatoes.
a. are
b. have been
c. were
d. is

The answer is D
The saucepan or the stockpot_is_ wide enough for all of the potatoes.

(IS) third person singular present of be which is meaning its happening now

Because it makes a lot more scene if IS is in that sentence 

(HAVE BEEN) means it was there but not anymore

(ARE) 2nd person singular present and 1st, 2nd, 3rd person plural present of be. For an example (Are you there) which does not make any scene for the The saucepan or the stockpot_are_ wide enough for all the potatoes.

(WERE) is like were you there or what are you wereing

So the answer is D. is
That is the best answer choice