Examine Assertion (A):
Assertion says: both rhombic sulphur and monoclinic sulphur exist as $\mathrm{S}_8$, while oxygen exists as $\mathrm{O}_2$.

• Rhombic sulphur and monoclinic sulphur are two allotropes of sulphur.
• In both these crystalline forms, the basic molecular unit is the puckered ring molecule $\mathrm{S}_8$.
• Oxygen commonly exists as diatomic molecules, $\mathrm{O}_2$.

Hence, Assertion (A) is correct.

Examine Reason (R):
Reason says: oxygen forms $p\pi-p\pi$ multiple bonds with itself and with small, highly electronegative elements like C and N, which is not possible for sulphur.

• Oxygen is a second-period element with small size, so effective sideways overlap of $p$ orbitals is possible.
• Therefore oxygen readily forms $p\pi-p\pi$ bonds, as in $\mathrm{O}_2$, $\mathrm{C=O}$, $\mathrm{N=O}$, etc.
• Sulphur is a third-period element and has larger atomic size. Effective $p\pi-p\pi$ overlap is poor, so it does not easily form stable multiple bonds like oxygen.
• Because of this, sulphur prefers catenation and single-bonded ring structures such as $\mathrm{S}_8$.

Hence, Reason (R) is also correct.

Check whether (R) explains (A):

• Oxygen exists as $\mathrm{O}_2$ because it can form a stable $\mathrm{O=O}$ multiple bond.
• Sulphur cannot form such strong multiple bonds due to poor $p\pi-p\pi$ overlap, so it prefers larger ring structures like $\mathrm{S}_8$.

Therefore, (R) is the correct explanation of (A).

Conclusion:
The correct option is $\boxed{D}$ i.e. both Assertion and Reason are correct, and Reason is the correct explanation of Assertion.