1. Magnetic field due to a semicircular arc

For a circular arc of angle $\theta$ at the centre,

For a semicircle, $\theta=\pi$, so

2. Field due to straight radial segments

The two semicircles are connected by radial segments. At the common centre $O$, for any radial segment,

Since $d\vec l$ is along $\hat r$, the cross product is zero. Hence, the straight connecting parts produce no magnetic field at $O$.

3. Net magnetic field at the centre

The current goes through the two semicircular arcs in opposite senses around the centre, so their magnetic fields at $O$ are opposite.

Thus,

Substitute $R_1=2\pi\,\text{m}$, $R_2=4\pi\,\text{m}$, $I=4\,\text{A}$:

Using

we get

So,

Hence,

4. Comparison with stored answer

Stored correct answer is $3$, but the magnetic field calculated from the standard semicircle formula is $1\times 10^{-7}\,\text{T}$.

So the stored answer appears to be incorrect unless the figure indicates both semicircles contribute in the same direction in a different current arrangement. For the usual loop made of two concentric semicircular arcs connected radially, the correct value is $1$.