Use suitable auxiliary verb to fill in the blank space:
Mother says you ______ to go to the market at once.

The presence of the infinitive marker "to" after the blank space narrows down the options. The phrase "have to" expresses obligation or necessity. "You have to go" is grammatically correct and fits the urgent context indicated by "at once." "Are to go" is also grammatically valid to express an instruction/arrangement, but "have to" strongly emphasizes the obligation coming from the mother's command. In standard syntax tests, "have" is preferred before an infinitive showing necessity.