The correct answer is "his chubby legs pumping"
Explanation:
In sentences, an absolute phrase is a set of words that act as a modifier as they refer to a noun. Besides this, absolute phrases can be identified because they are separated from the main sentence by using a comma and these are not essential to the meaning of the sentence.
In the case of the sentence "The baby toddled across the room with his first wobbly steps, his chubby legs pumping" the section "his chubby legs pumping" acts as an absolute phrase because this describes or modifies "the baby"; also, this is separated using a comma and it is not essential as it can be removed.
