Hii,

To calculate this we need to take the following steps:

For a A plane EM wave traveling in the x-direction is of the form

E (x,y) = 10j cos[(6x+8z)]

B( x,y,z) = B0 cos[(6x+8z)] - eq 1

where B0 = E0/C

Now since E is in the direction of j

Similarly the direction of wave is along αi+βk

The direction of propagation is the direction of E x B.

Hence,

B = 1/C(6k-8i) cos [(6x+8z-10ct)]

B is in direction B = 1/C( 6k-8i)

and for this option E x B is along αi+βk.

Hope this helps.