a
Let us first decide the standard direction on the plane of paper. current carries from North to south direction and point $\mathrm{B}$ is placed right side of wire. according to right hand palm rule, direction of magnetic field is upward . [ see figure]
magnitude of magnetic field, $|B|=\frac{\mu_{0} 2 I}{4 \pi r}$
$=10^{-7} \times \frac{2 \times 50}{2.5}$
$=4 \times 10^{-6} T$
hence, in vector form magnetic field $B=4 \times 10^{-6} \mathrm{k}$ T
