Option(a) is the correct answer because the elements in option a has completely filled their valence shell which is one of the property shown by noble gases and also these two are colourless and odourless.

Noble Gases and Chemical Reactions

Noble gases are a group of elements located in Group 18 of the periodic table. They are also known as inert gases because they have a stable electron configuration and rarely participate in chemical reactions. This is due to the fact that noble gases have a full outermost electron shell, making them highly stable and unreactive.

Explanation:

1. Noble Gases
- Noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn).
- They are all characterized by having a full outermost electron shell, which gives them high stability.
- This electron configuration is achieved by having either 2 or 8 valence electrons, except for helium which has only 2.

2. Inert Nature
- Due to their full outermost electron shell, noble gases have no tendency to gain, lose, or share electrons with other elements.
- This stability makes them highly unreactive and unlikely to participate in chemical reactions.
- Noble gases are often referred to as inert gases because of their lack of reactivity.

3. Exception - Kr and Rn
- While noble gases are generally unreactive, there are two exceptions - krypton (Kr) and radon (Rn).
- These two elements have larger atomic sizes compared to the others in the group, which allows for some limited reactivity.
- Krypton and radon can form compounds under certain extreme conditions, such as high pressures and temperatures.

4. Participating Elements
- Among the given options, only krypton (Kr) belongs to the noble gases.
- Helium (He) and neon (Ne) are considered completely inert and do not participate in any chemical reactions.
- Therefore, the correct answer is option 'A,' which states that Kr participates in a chemical reaction.

Summary:

Noble gases are generally unreactive due to their stable electron configurations. However, krypton (Kr) and radon (Rn) can form compounds under extreme conditions. Among the given options, only krypton (Kr) participates in chemical reactions, making option 'A' the correct answer.