According to the question

We know the formula,

Average of the cube's 1st 'n' natural numbers= $\frac{\mathrm{n}(\mathrm{n}+1{)}^2}{4}$

where, n=5

∴ Required average =$\frac{5{\left(5+1\right)}^2}{4}=\frac{5\times 36}{4}$

= 5×9= 45

Hence, Option B is correct.