The non-metal that reacts and catches fire on exposure to air is (a) phosphorus.
Explanation: Phosphorus is a highly reactive non-metal that readily ignites when exposed to oxygen in the air, making it necessary to store it underwater to prevent spontaneous combustion.
Why other options are incorrect:
(b) nitrogen:
Nitrogen is relatively inert and does not readily react with oxygen under normal conditions.
(c) sulfur:
Sulfur reacts with oxygen to form sulfur dioxide, but it does not spontaneously catch fire like phosphorus.
(d) hydrogen:
Hydrogen is flammable when mixed with oxygen, but it requires an ignition source to react. It does not spontaneously combust in the air like phosphorus.