Two infinite plates carry $j$ ampere of current into the page per unit width of the plate as shown. $B_P$ and $B_Q$ represent magnitude of magnetic field at points P and Q respectively.

 

 

 

 

 

 

 

 

Consider only one plate as shown in figure. The field on both sides of plate is shown in figure. Applying Ampere’s circuital law to the contour C, we get,

  $2BL={\mu }_{0}j\left(\int dl\right)\Rightarrow B=\frac{{\mu }_{0}j}{2}$
Superimposing, the field due to two plates, we get that at P,  both fields cancel each other and at Q, they add to give $B_Q={\mu }_{0}j$

Option (C) is wrong, Option (D) is correct. Option (A) is correct, Option (B) is wrong.