The verb in this given sentence is c. irregular. The verb "cost" is an irregular verbs because it doesn´t follow the regular pattern -ed,-d or -ied to form past tenses. It just remains the same, making it irregular, such as the verbs "cut", "put", "burst", "bid".
It is worth mentioning that irregular verbs are considered strong ones; the regular ones (those ending in -ed, -d or -ied) are considered weak verbs.
