Assertion (A) : The empirical mass of ethene is half of its molecular mass.Reason (R) : The empirical formula represents the simplest whole number ratio of various atoms present in a compound

The correct option is A Both A and R are true and R is the correct explanation of A.
Analysing Assertion and reason to understand the answer.

Analyzing the assertion

Empricial mass of Ethene =
( Molecular mass of ethene) / (2)
Molecular formula of ethene = $C_2{H}_4$
Molecular mass of ethene = $28u$
Empirical formula of ethene = $C{H}_2$

Empricial mass of ethene = $14u$

$\Rightarrow Empiricalmass=\frac{1}{2}X\left(Molecularmass\right)$
Molecular formula = (Empirical formula)n
for ethene: molecular formula = $C_2{H}_4$
Empirical formula = $C{H}_2$

Or molecular formula = $(C{H}_2{)}^2$

So, the ratio varous atom $\left(C/H\right)$
empirical formula $\left(C{H}_2\right)$
$\left(C_2{H}_4\right)$ is 1:2,

$i.e,$ simplest whole ratio

Conclusio

Thus, the empirical formula represents the simplest whole number ratio of various atoms present in a compound.

The given assertion statement is correct and
the given reason statement is also correct.

And the reason is the correct explanation of A.Hence, correct answer is first option (i.e. Option (A)