When an atom decays by ß decay, its mass number remains the same.
Tritium beta decays into Helium-3 (isotope of Helium which has a mass number 3). You may have figured it out already as all three options have He as a product.
It may seem that HTO will form HeOH as both are single molecules. However, He is a noble gas and does not react with or form molecules with other elements. Therefore, HeOH cannot be a product.
Tritium will form an He atom on its own.
Let us look at equations 2 and 3 and try to balance them. We can substitute He with T as we know T will form He.
4HTO -> 2H₂O + O₂ + 4T
HTO -> H₂O + H₂ + T (not possible to balance)
If we try to balance both equations, you will realise that it is not possible to balance the second equation.
Hence only equation 2 is possible. B is the answer.