The answer is: D. Sodium (Na).
Beta decay is radioactive decay in which a beta ray and a neutrino are emitted from an atomic nucleus.
There are two types of beta decay: beta minus and beta plus.
In beta plus decay (atomic number Z is decreased by one), a proton is converted to a neutron and positron and an electron neutrino, so mass number does not change.
In this example, magnesium (atomic number 12) is converted to dosium (atomic number 11).