The correct answer is A. When the subject of a sentence is singular, add "_s" to the base form.
An easy example is "to exist", the base form is "exist" and A strawberry exists shows that you add s when a singular subject is used.
As·ser·tion (ə-sûr′shən)n.1. The act of asserting.2. Something declared or stated positively, often with no support or attempt at <span>proof.</span>
