Answer:
I believe line B is iambic:
B. I went to Boston Sunday morning.
Step-by-step explanation:
Iamb is a sequence consisting of an unstressed syllable followed by a stressed one. If that happens four times, for instance, it is called a iambic tetrameter. If it repeats five times, a iambic pentameter, and so on. Therefore, we are looking for the line that shows a pattern of unstressed-stressed. In that case, the only possibility is letter B. See what happens when we highlight the stressed syllables in line B:
B. I went to Boston Sunday morning.
We have iambs, an unstressed syllable followed by a stressed one. We can conclude, thus, that letter B is the best option.
Just to prove how the same does NOT happen in the other options, let's highlight their stressed syllables as well:
A. Pirouette for the folks in the back.
C. Martin isn't ever going fishing now.
D. Music is harmony, rhythm, and melody.