An antenna emits an electromagnetic wave. the electric field lines at a certain instant in time are shown. at point a, what is the direction of the magnetic field?

The image is missing, but let's try to answer it.

When we have a plane electromagnetic wave that moves in the K direction, we aways must have that the electric field and the magnetic field are orthogonal.

Where the magnetic field direction can be calculated with the equation:

B = KxE

this is a cross product, to find the direction of B, you can point with your hand in the direction of the first vector, and close your hand in the direction of the second vector, the place at where your thumb is pointing is also the direction of B.

This means that if we have that the wave moves in the X direction, and E is in the Z direction, we will have that B is in the XxZ = Y direction.

it would be kevlar ^^