They had just started to cook dinner when their friends arrived at the door with some delicious take-out food.

which word is the adverb modifying a verb?

The correct answer is Just

Explanation:

In sentences, an adverb is a word that modifies or describes verbs in terms of time, frequency, place, mode, etc. This type of words can be recognized because some of them end in "-ly" and also most of them are commonly placed near to the verb. In the sentence "They had just started to cook dinner when their friends arrived at the door with some delicious take-out food", the word that acts as an adverb is "just" as this defines or modifies the main verb "had started to cook" by showing time; also, although this adverb does not end in "-ly" is placed as part of the verb it modifies.

"the sandy walls seemed to collapse in on themselves"

and

"the bloodshot waters seemed to have no bottom"