Correct Answer - C
Magnetic field at centre due to a square of side `a:`
`B=4[(mu_(0))/(4pi) i/((a//2))[sin 45^(@)+sin45^(@)]] =(2sqrt(2) mu_(0)i)/(pia)`
For the case in fig.
`B_("net")=(2sqrt(2) mu_(0)i)/(pi)[1/a-1/(2a)+1/(3a)-1/(4a)+.....]`
`=(2sqrt(2)mu_(0)i)/(pia)[1-1/2+1/3-1/4+.....]`
`=(2sqrt(2)mu_(0)i)/(pia)(ln 2)`