Which label belongs in the area marked x? current magnetic field solenoid force on wire

when we need to represent the magnetic field in a specific region where it is given that magnetic field lines are perpendicular to the plane and going inside the plane then we always use the concept of representation by "X"

here this symbol represents that field lines are going into the plane

it is also the uniform magnetic field always

so the correct answer would be

Magnetic Field

real, smaller and upside-down.   so the answer should be b.