In experimental arrangement to study photoelectric effect, here M, N,…

The correct answer is Option (3) → (D), (C), (B), (A)

In the photoelectric tube the light enters through a window, falls on a sensitive plate inside an evacuated tube, and the current is controlled and reversed through a commutator. Reading the labels of the figure:

M points to the plate C from which electrons are emitted — it is the (D) photosensitive plate.

N points to the opening through which the radiation S enters — it is the (C) quartz window, which lets ultraviolet light through.

O points to the envelope of the tube — it is the (B) evacuated glass tube, so that the electrons are not stopped by collisions with air molecules.

P points to the switching arrangement joining the plate and collector to the battery — it is the (A) commutator, which reverses the polarity of the applied potential.

So M, N, O, P are (D), (C), (B), (A), which is Option (3).

NCERT (2026-27):
Experimental Study of Photoelectric Effect – “Figure 11.1 depicts a schematic view of the arrangement used for the experimental study of the photoelectric effect. It consists of an evacuated glass/quartz tube having a thin photosensitive plate C and another metal plate A.”

Experimental Study of Photoelectric Effect – “… A transparent quartz window is sealed on to the glass tube, which permits ultraviolet radiation to pass through it and irradiate the photosensitive plate C.”

Experimental Study of Photoelectric Effect – “… The polarity of the plates C and A can be reversed by a commutator.”