Which sentence contains an inappropriate shift in tense?
a. i am reading a poem that was written thousands of years ago.
b. i finished painting the fence, but barbara is still repairing the gate.
c. hummingbirds are hovering over the geraniums while caterpillars munched leaves.

C. Hummingbirds are hovering over the geraniums while caterpillars munched leaves.

This is because the tense of the hummingbirds is in the "ing" form, while the tense of the caterpillars is in past tense, or the "ed" form.  Because these tenses do not agree, it is an inappropriate shift in verb tense.