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.

Refute it, showing why it is mistaken—an apparent but not real problem; acknowledge its validity or plausibility, but suggest why on balance it's relatively less important or less likely than what you propose, and thus doesn't overturn it; concede its force and complicate your idea accordingly—restate your thesis in a more exact, qualified, or nuanced way that takes account of the objection, or start a new section in which you consider your topic in light of it. this will work if the counterargument concerns only an aspect of your argument; if it undermines your whole case, you need a new thesis.