The correct option is D Rs. 40 lakhs
Let the C.P. of the land be x
Gain %=x%
Gain =x% of x= x2100
S.P. = C.P. + gain
S.P. =56 lakhs
Therefore, x+x2100=56
100x+x2=5600
x2+100x−5600=0
(x+140)(x−40)=0
x=−140 or 40
Price cannot be negative,
So, cost price of the land = Rs. 40 lakhs.