Carbon-14 forms nitrogen-14 by
a. alpha decay.
b. beta decay.
c. gamma decay.
d. none of the above

B. Beta decay

Beta decay occurs when a high-speed electron is lost from the atom, converting one of the atom's neutrons into a proton. The mass number of the atom then remains the same but its atomic number is increased by one.

This is the decay taking place because the transition from carbon-14 to nitrogen-14 involves an increase in atomic number by 1.

The correct answer is d