Number of turns on the circular coil, n = 100

Radius of each turn, r = 8.0 cm = 0.08 m

Current flowing in the coil, I = 0.4 A

Magnitude of the magnetic field at the centre of the coil is given by the relation,

|B|= (μ0/4π) 2πnl/r

Where , μ0 = Permeability of free space = 4π x 10⁻⁷ Tm A⁻¹

So,

|B|= (4π x 10⁻⁷)/4π     x   (  2π  x 100 x 0.4 )/r  =3.14 x 10⁻⁴ T

Hence, the magnitude of the magnetic field is 3.14 x 10⁻⁴ T.