Two long parallel wires are at a distance $2d$ apart. They carry steady equal currents flowing out of the plane of the paper as shown. The variation of the magnetic field $B$ along the lines is given by

 

If the current flows out of the paper, the magnetic field at points to the right of the wire will be upwards and to the left will be downwards as shown in figure.

Now, let us come to the problem.

Magnetic field at $C=0$

Magnetic field in region $BX\text{'}$ will be upwards (+ve) because all points lying in this region are to the right of both the wires.

Magnetic field in region $AC$ will be upwards (+ve), because points are closer to $A$, compared to $B$. similarly, magnetic field in region $BC$ will be downwards(-ve).

Graph (b) satisfies all these conditions. Therefore, correct answer is (b).