Answer:
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 "<em>Is </em><em>Mar</em><em>tin </em><em>Lew</em><em>is </em><em>Go</em><em>ing </em><em>Mad, </em><em>my </em><em>dear</em><em>?"</em>
Which helps to identify which syllables are stressed and unstressed, following iambic pentameter.