Let 'C' be the cost of original diamond and its weight 'w'

C = k.w^{2 } .... (given)

Now, w1 : w2 : w3 : w4 = 1x : 2x : 3x : 4x

w = w1 + w2 + w3 + w4 = 10x

From the given condition,

k.(10x)^{2} = k.(1x)^{2 }+^{ }k.(2x)^{2 }+ k.(3x)^{2} + k.(4x)^{2 }+ 70000

k.100x^{2} = k.30x^{2} + 70000

k.x^{2} = 1000

So the original cost of diamond is k.(10x)^{2} = 100000 = Rs. 1.0 lakh

**C is the answer.**