Final answer:
The direction of the magnetic field causing protons to move in a circle can be determined by the right-hand rule for magnetic force. For protons moving in a counterclockwise direction, the magnetic field must point upwards (out of the plane of the circle).
Step-by-step explanation:
The direction of the magnetic field in which protons move in a circle can be determined using the right-hand rule for magnetic force. When protons, which are positively charged particles, move in a magnetic field, they experience a magnetic force that is perpendicular to both their velocity and the magnetic field. This force causes them to move in a circular path. According to the right-hand rule, if you point your thumb in the direction of the proton's velocity and your fingers in the direction of the magnetic field, the force (and therefore the direction of the resulting circular motion) will come out of your palm. Thus, for a proton moving in a counterclockwise direction in a circular path when viewed from above, the magnetic field must point upwards (out of the plane of the circle).