The correct option is A μ0l2Rinterior
Magnetic field by Biot Savart law,
dB=μ04πidlsinθr2
Due to the straight wires, θ=0 i.e. it lies on the same line as the centre.
Due to curved portion, lets consider infinitesimal length dl where dl and R are always at 90 degree.
Hence dB=μ04πidlR2
Integrating from 0 to πR
dB=μ0i4R