Question

Which one of these lines uses iambic pentameter

Answer 1

Iambic pentameter is fives sets of iambs, or one unstressed followed by one stressed.

The only choice that matches that description is option A. 
The others all start with a stressed syllable, so they can not be iambs.

Hope this helps =)

Answer 2

A pentameter is a line in verse consisting of five metrical feet.

Iambic pentameter is one of the most commonly used meters in English poetry. It is a metrical foot in poetry that refers to the number of syllables in a line and the emphasis placed on them. According to this pentameter, an unstressed syllable is followed by a stressed syllable.

According to this, the line that uses iambic pentameter is option A "Is Martin Lewis going mad, my dear?" because it presents an unstressed syllable followed by a stressed one.