Magnetic Effect of Current-2

(D)

using biot and savart law dB=[μo*idl(cross)r]/4πr
diretion of dl is along the direction of motion of positive charge or opposite to the motion of negative charge.
so direction of B is out of the plane for all three charges.(using right hand thumb rule)
using i=dq/dt and dl/dt=v and integrating both sides we get,
B=μoqv(cross)r/4πr
therefore for the three charges we can add B algebrically(as it has same direction due to all the charges)
on adding we get,
B=μoqv4Ï€a²[q+q+2q]
B=4μoqv4Ï€a²Out of the plane.