11H + 31H → ___A. 42He B. 42H C.43He D. 32He

The given equation represents a nuclear fusion reaction where two isotopes of Hydrogen (1H and 3H) combine to form a new element.

Step 1: Identify the atomic numbers and mass numbers of the reactants. Hydrogen (H) has an atomic number of 1. The superscript numbers 1 and 3 represent the mass numbers of the hydrogen isotopes.

Step 2: In a nuclear reaction, both the total atomic numbers and the total mass numbers are conserved. This means the sum of the atomic numbers on the reactant side (left) must equal the sum of the atomic numbers on the product side (right). The same applies to the mass numbers.

Step 3: The atomic number of Hydrogen (H) is 1. So, the total atomic number on the reactant side is 1 (from 1H) + 1 (from 3H) = 2. The total mass number on the reactant side is 1 (from 1H) + 3 (from 3H) = 4.

Step 4: Therefore, the product must have an atomic number of 2 and a mass number of 4 to conserve both atomic and mass numbers.

Step 5: The element with an atomic number of 2 is Helium (He). So, the product is a Helium isotope.

Step 6: Looking at the options, the only one that fits these criteria is 42He.

So, the correct answer is A. 42He.