Which is a sentence fragment?

a. Balancing on the beam, she didn't fall.

b. After dinner, we watched TV.

c. Rosa attended the lecture.

d. Answered your e-mail this morning.

Sentences a, b, c, each have a subject and a verb: that's a good sign that they might be sentences.

Sentence d in contrast does not have a subject. Who ansered the email? we don't know!

it's probably the speaker but it could be anyone

the correct answer is d!