Correct Answer - A
a. If we take a small strip of `dr` at distance `r` from centre, then number of turns in this strip would be
`dN=(N/(b-a))dr`
Magnetic field due to this element at the cenntre of the coil will be
`dB=(mu_)(dN)I)/(2r)=(mu_0NI)/((b-a)) (dr)/(2r)`
`:. Bint_(r=a)^(r=b)dB=(m_0NI)/(2(b-a))ln(b/a)`
`:. current answer is a.