A contract containing an undertaking to do an impossible act, is

void ab initio
voidable
valid
illegal

The correct answer is A. void ab initio.

A contract containing an undertaking to do an impossible act is void ab initio, which means that it is not legally binding from the very beginning. This is because it is impossible for the parties to fulfill their obligations under the contract.

Option B, voidable, is incorrect because a voidable contract is one that is valid but can be canceled by one

or both parties. Option C, valid, is incorrect because a contract that is impossible to fulfill is not legally binding. Option D, illegal, is incorrect because a contract that is impossible to fulfill is not necessarily illegal.

Here are some examples of impossible acts:

  • Promising to fly to the moon.
  • Promising to turn lead into gold.
  • Promising to bring someone back from the dead.

If you are ever unsure whether a contract is valid, it is always best to consult with an attorney.