At this intersection, who must go first?

Correct answer: The truck.

At an intersection without signs, whoever has no traffic approaching from their right goes first. The truck approaches from the red passenger car’s right, and no one is approaching from the truck’s own right, so it goes first. The passenger car waits for the truck, and the moped has the passenger car on its right.

Real Decreto 1428/2003, de 21 de noviembre, por el que se aprueba el Reglamento General de Circulación