Question

Out of white phosphorus and red phosphorus, which one is more reactive and why?

Answer 1

One of the phosphorus is more reactive than the other one because it has an angular strain in the $P_4$ molecule where all the angles are 60° only. While there is no such angular strain in the other molecule. The reactive one must be stored under water to avoid its oxidation.

Complete step by step answer:
White phosphorus is the one which is more reactive than red phosphorus as white phosphorus has angular strain in $P_4$​ molecules having all the angles of only $60^o$. While there is no such angular strain in Red Phosphorous.
Although Red Phosphorus contains more atoms linked with each other in a network than the white phosphorus does, which actually makes it much more stable.
Hence, White phosphorus is the one which is least stable, but the most reactive, also the most volatile, the least dense, and the most toxic of the allotropes. It actually changes to red phosphorus, by a transformation which is accelerated by light and heat.

ADDITIONAL INFORMATION:
White phosphorus is actually a toxic substance formed from phosphate containing rocks. White phosphorus is used industrially in manufacturing the chemicals which are used in fertilizers, food additives, and also for cleaning compounds. It has been used in the past like a pesticide and also in fireworks.
Red phosphorus is actually one of the most common allotropes of phosphorus and it is generally considered to be a derivative of the $P_4$ molecule. It is present in an amorphous (or non-crystalline) network of phosphorus atoms. It is thereby found to be more stable than white phosphorus (which is another naturally occurring phosphorus allotrope).

Note:
As mentioned in the answer, White phosphorus is the one which is least stable, but the most reactive, also the most volatile, the least dense, and the most toxic of the allotropes. Also there is no such angular strain in Red Phosphorus.