Why is argon less reactive than sulphur?

Argon is a noble gas, while sulfur is not. And this is what makes sulfur more reactive as compared to argon. 

To react, an atom needs electrons that can be shared or donated or gained. That is only possible if the valence electron shell is not fully filled. If the atom has achieved a fully filled electronic configuration, it will not react any further.

Sulfur has an atomic number of 16 and an electronic configuration of 1s2, 2s2, 2p6, 3s2, 3p4. This means that it needs 2 more electrons or 4 less electrons for a fully filled configuration. This will make sulfur reactive.

In comparison, argon has an atomic number of 18 and an electronic configuration of 1s2, 2s2, 2p6, 3s2, 3p6. That means that no electrons are available for sharing or exchange and that argon atom already has a fully filled configuration. This makes argon almost inert.

Thus, sulfur is more reactive as compared to argon.

Hope this helps.