What is the best way to correct the error in this sentence?

Either Tina or Mark buy groceries each Saturday morning.

a) Change the verb to 'buys'.

b) Delete the first word in the sentence.

c) Delete the conjunction.

d) Change the verb to 'are buying'.

To me, the answer i would pick is B because the word "or" already shows either one of them. The other choices just don't seem right.

For A, you couldn't have "buys" and immediately have "groceries" with plural. C would just be weird. D shows that they're continuing or a gerund, therefore showing that it is a continuous action, but it doesn't show a habit.