is this sentence in active voice or passive voice? some adult children may still be covered by their parents…

Brief Explanations:

In the sentence "Some adult children may still be covered by their parents' medical insurance policies", the subject "some adult children" receives the action of being covered. In passive - voice sentences, the subject is the recipient of the action, and the preposition "by" is often used to indicate the doer of the action (in this case, "their parents' medical insurance policies").

Answer:

passive voice