Final answer:
The element that results from neptunium-236 undergoing beta decay is plutonium-236 (Pu-236), caused by an increase in the atomic number while the mass number remains unchanged.
Step-by-step explanation:
During beta decay, an unstable nucleus transforms by converting a neutron into a proton and emitting an electron (called a beta particle). This increases the atomic number by one while the mass number remains the same. Based on the information, neptunium-236 (Np-236) has a high neutron-to-proton ratio and is likely to undergo beta decay. After beta decay, the resulting element would have an atomic number increased by 1, which means the element that forms when neptunium-236 undergoes beta decay is plutonium-236 (Pu-236).