Rockwell had been attending the new york school of art when he dropped out and enrolled at the national academy of design.

what is the tense of the underlined verb?

simple past

past perfect

past progressive

past perfect progressive

Based on the given sentence above, the underlined verb is had been attending. So this verb's tense is PAST PERFECT PROGRESSIVE. This tense is formed using the words "HAD" and "BEEN" plus the -ing form of the verb. This tense indicates a continuous or progressive action that has been completed at some point in the past. Hope this answer helps.