To understand what determines the reactivity of elements in chemical reactions, we need to delve into basic atomic and molecular chemistry.

1. Role of Electrons in Reactivity:
   • The reactivity of an element is largely determined by its ability to gain or lose electrons. This is because chemical reactions often involve the transfer of electrons.
   • For metals, reactivity often increases down a group as electrons can be lost more easily due to increased atomic size. For instance, alkali metals like potassium are more reactive than lithium.
   • For non-metals, reactivity often decreases down a group because the ability to attract electrons becomes weaker. Halogens are a classic example, with fluorine being more reactive than iodine.
2. Why Other Options Are Incorrect:
   • The option stating that "Reactivity remains constant for all chemical elements" is incorrect because elements behave differently under varying conditions.
   • The notion "Reactivity independent of electron transfer in reactions" is incorrect as electron transfer is a fundamental aspect of chemical reactivity.
   • The option "Reactivity depends only on nuclear properties of atoms" overlooks the electrochemical aspect where electron interactions are key.

Conclusion: The reactivity of elements in chemical reactions predominantly depends on the ease of electron loss or gain, aligning with the correct answer: "Reactivity depends on ease of electron loss or gain."