An alternative promise, one branch of which is legal and the other is illegal

There can be such a contract vide Section 23 of the Indian Contract Act.

There can be such a contract, but the contract will only be for the legal set and not for the illegal set of promises vide Sections 57 and 58 of the Indian Contract Act.

The contract as a whole will be enforceable at the option of the buyer and not the seller.

There can be a contract of two set of promises, with one set of promises being legal and the other set of promises being illegal, provided both the set of promises are a commercial contract and the monetary value of the illegal set of promises is reduced from the value of the legal set of promises, and enforcement is only for the net value.