Select every verb that keeps one single form throughout, so that its base, past simple and past participle are identical (as in cast — cast — cast). Read cost in the price sense of what something charged a buyer.

Erklärung

Four of these verbs never change: the gearbox cost a fortune, they cut the rope, we let them in, the fire spread quickly — every one of those past forms looks like the base. Sit and meet do change, because sit — sat — sat and meet — met — met share the past simple and the participle with each other but not with the base, which is precisely what the one-form group avoids.