Final answer:
Fluorine releases the most energy when gaining an electron.
Step-by-step explanation:
Of the main group elements, fluorine has the highest electronegativity (EN = 4.0), indicating that it has a high tendency to gain electrons from other elements. Fluorine belongs to the halogen group (Group 17) and gains electrons most readily, releasing the most energy in the process of forming halide ions. Therefore, the element that releases the most energy when gaining an electron is fluorine (option C).