Which of the following is a sentence fragment?
a. marina is a fast runner and skilled basketball
b. marina, and excellent basketball player
c. marina has a very strong basketball skills
d. marina, a great basket ball player, scored 26 pints today.

Assuming you mean to write "basketball player" in the first option, "b. marina, and excellent basketball player" would be a sentence fragment since there is no verb--it is only describing Marina. 

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.