When 3 methyl heptane is treated with h2so4 for sulphonation, it is said to be racemised after the rxn.

i dont understand how, earlier also it was an optically active compund and even now it is the same.

You are absolutely write , the compound formed is also optically active but in sulphonation, there will be two types of product formed and those are called enantiomer. Here at the chiral centre, in one enantiomer, the Me group would be upward and in other one it would be downward.
The product contains these two enantiomers in 1:1 ratio.
As these two enantiomers, rotate light equally but in opposite direction so overall the rotation of light is zero due to equal amount of both the products and hence this mixture is called as racemic mixture (having 0 net rotation).