The correct option is A Rs. 50 lakhs
Let the C.P. of the land be x
Gain %=x%
Gain =x% of x= x2100
S.P. = C.P. + gain
S.P. =75 lakhs
Therefore, x+x2100=75
100x+x2=7500
x2+100x−7500=0
(x+150)(x−50)=0
x=−150 or 50.
Price cannot be negative,
So, cost price of the land = Rs. 50 lakhs.