CAT = XAT

Implies C changes to X all else remains the same.

Now we know that C is the third letter and X is the 24th letter in the alphabet set. We can assume that the X is obtained by simply multiplying the C's 3 by 8 to get 24 which is X's position in the alphabet set.

Similarly we can check the rest of the cases given here and my assumption will hold true

So based on that HAT should be LAT.

Note that while multiplying and obtaining the position of the target alphabet if the product crosses 26 then the counter should reset to 1.