Radioactive elements will continue to decay until a. they have a stable nucleus b. they have a full octet of electrons c. they have a neutral electrical charge d. they have the same number of electron and protons

A). Until they have a stable nucleus.

A. They have a stable nucleus

Explanation:

Radioactivity is a spontaneous disintegration or decay of an unstable nuclei into a more stable nucleus along with the release of radiation in the form of elementary particles like alpha, beta or gamma particles or in the form of x-rays. The elements that exhibit this property are called radioactive elements or radioisotopes. They tend to lose their radioactivity over time which is usually expressed in terms of their half lives (t1/2) i.e. the time taken to decay into half of their original amount.

Isotope of Uranium with atomic mass of 235 amu is a radioactive element which is used in nuclear reactors to produce energy. It decays into a stable nucleus of Barium(Ba) and krypton(Kr) along with the release of 200 Mev energy.

₉₂U²³⁵ + ₀n¹  →   ₅₆Ba¹⁴¹ + ₃₆Kr⁹² + 3₀n¹ + 200 MeV energy

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.