The correct answer is D. Forget about the plan
Step-by-step explanation:
A line, sentence, or verse is iambic if this is composed of iambs, which are units with an unaccented syllable and an accented syllable. Additionally, in language accented or stressed syllables are those that you pronounce with more emphasis and due to this, these are also "long" syllables.
In the case of the line "Forget about the plan," this uses iambs because the line is composed of unstressed/stressed syllables "Forget about the plan" (stressed syllables are in bold). Indeed, in this line, there are three iambs, which is known as iambic trimeter.