Whenever a stream of electrons collides with a stream of photons, in this collision, which of the following is not conserved?
(A) Linear Momentum
(B) Total Energy
(C) No. of photons
(D) No. of electrons

Hint We are given the situation where a stream of electrons and photons collide and are asked to evaluate which of the parameters is not conserved. Thus, we will discuss each and every parameter one by one and the change in them due to the collision. Finally, we will come up with a selection of the correct option out of the given four.

Complete Step By Step Solution
Firstly,
When the two streams collide, the electrons and photons might change its velocities. Either it might increase or decrease. When the electrons and the photons collide, the electrons absorb the photons and thus might change its mass. Further, the linear momentum is not conserved as the linear momentum depends on the initial and final mass and the velocity of the system.
Then,
The total energy remains nearly the same as the collision of the streams is elastic. Thus, the total energy of the system is conserved.
Then,
As the electrons absorb the photons while colliding. Thus, the number of photons changes. Hence, the no. of photons is not conserved.
Finally,
The electrons absorb the photons while colliding and thus the no. of electrons is conserved.

Hence, the correct options are (A) and (C).

Note We have considered all the parameters one by one for evaluating the correct option(s). We could have directly jumped into a conclusion. But a student should be very cautious about committing any sort of mistake and hence has to discuss through all the parameters.