The answer to your question would be that in the line in question from "Sonnet 18" there are five iambs. That is, the right option would be B.
An iam, also known as iambus, is a metrical foot consisting of one unstressed syllable followed by one stressed syllable. The iambic pentameter is an iambic verse used by Shakespeare, his colleagues, and imitators, and by other romantic poets throughout the ages.
