Which one of these lines uses iambic pentameter?
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a. is martin lewis going mad, my dear?
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b. "watch out! " the young woman cried.
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c. vicki used to be a child actress but grew up.
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d. honeydew is the worst melon of all melons.

A. Is Martin Lewis going mad, my dear?

Explanation:

Iambic pentameter is a line consisting of ten syllables, which leaves A as the only possible option as B has 7, C has 13, and D has 12.

To dive further into it though, let's suppose that there were two potential answers with 10 syllables. Iambic pentameter is the use of an unstressed syllable followed by a stressed syllable, repeated five times. Naturally, for the most part, this is how language forms, and we often learn a lot about this when learning Shakespeare (because his works are almost entirely constructed around iambic pentameter).

In "A. Is Martin Lewis going mad, my dear?"

We can read is as "Is Martin Lewis Going Mad, my dear?"

Which helps to identify which syllables are stressed and unstressed, following iambic pentameter.