Hey Sumit.

After Class 12th from Science stream you will be eligible to do both Law as well as BA. There is no restriction based on your stream in class 12th. The choice between the two should be based on where your area of interest lies. If you want to pursue law, then CLAT and other entrances for law would be a better option as joining law after class 12th you'll be able to complete it in 5 years only. And if you do BA first and then go for law separately then you'll take 6 years in total 3 for each. After 12th directly you'll have better and wider options for law colleges. So you can prepare for CLAT and keep BA as a backup option.

Best of luck