The number of turns on the coil $(n)$ is given as 100
The radius of each turn $(r)$ is given as $8 \mathrm{~cm}$ or $0.08 \mathrm{~m}$
The magnitude of the current flowing in the coil (I) is given as $0.4 \mathrm{~A}$
The magnitude of the magnetic field at the centre of the coil can be calculated by the following relation:
$|\bar{B}|=\frac{\mu_{0} 2 \pi n I}{4 \pi r}$
where $\mu_{0}$ is the permeability of free space equal to $4 \pi \times 10^{-7} T m A^{-1}$
hence,
$|\bar{B}|=\frac{4 \pi \times 10^{-7}}{4 \pi} \times \frac{2 \pi \times 100 \times 0.4}{0.08}$
$=3.14 \times 10^{-4} T$
The magnitude of the magnetic field is $3.14 \times 10^{-4} T$.