Which of the following contains only one element?

$A$ mixture of $NH_4Cl$ and $NaCl$ can be separated by

$A$ mixture of iodine and sodium sulphate is separated by:

Which of the following substances consists of only one element?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A:$ $A$ mixture contains benzoic acid and naphthalene. The pure benzoic acid can be separated out by the use of benzene.
Reason $R:$ Benzoic acid is soluble in hot water.
In the light of the above statements,choose the most appropriate answer from the options given below.

What is an element?