Assertion: White phosphorus is more reactive than red phosphorus
Reason: It readily catches fire in air to give dense white fumes of ${P_4}{O_{10}}$.
(A) Both Assertion and Reason are correct and Reason is the correct explanation for Assertion.
(B) Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion.
(C) Assertion is correct but Reason is incorrect.
(D) Both Assertion and Reason are incorrect.

Hint:Let's start by discussing whether the association is right or wrong.
Phosphorus is a chemical element with an atomic number 15 and a symbol P. Elemental phosphorus mostly exists in two major forms, white phosphorus and red phosphorus. But phosphorus is a highly reactive element, phosphorus never exists as a free element on Earth.

Complete step by step solution:
> The assertion in the given problem states “White phosphorus is more reactive than red phosphorus”. This assertion is true.
> The reason for this behavior lies in the structure of both white phosphorus and red phosphorus. They have the following structures
> As you can see that in red phosphorus 4 atoms are linked together and are also linked to other such consecutive groups of atoms, whereas, in case of white phosphorus the 4 atoms are only with each other and act as individual units. This crosslinking in red phosphorus makes it much more stable than white phosphorus.
> Additionally, in white phosphorus there is a high strain on the bonds because of its discreet packaging, whereas the strain is quite less in the case of red phosphorus.
Hence, the assertion is true.
> Now the statement for reason is “It readily catches fire in air to give dense white fumes of ${P_4}{O_{10}}$”
- The reaction of white phosphorus with air is shown below
${P_4}(s) + {O_2}(g)\xrightarrow{{}}{P_4}{O_{10}}(s)$
Hence, the reason given is a true statement, but as we saw above has no relation to the assertion.
Hence, option B is the correct choice.

Note:In such types of problems, you can skip writing a balanced equation for reactions as we did in the reaction ${P_4}(s) + {O_2}(g)\xrightarrow{{}}{P_4}{O_{10}}(s)$, as the purpose of showing this reaction is only to show the final product. But in other problems, especially in lab work it will be wise to write a balanced equation for every reaction. Example${P_4}(s) + 5{O_2}(g)\xrightarrow{{}}{P_4}{O_{10}}(s)$.