Which statement about the discovery of electrons is true?
a. electrons were discovered by experimental chemists.
b. electrons were discovered after the tv tube was invented.
c. electrons were discovered in a vacuum tube.
d. electrons were discovered when an electric current was passed through gases at low pressures.

The true statement about the discovery of electrons is :

C. Electrons were discovered in a vacuum tube

Explanation:

In 1870, Sir William Crookes developed the first cathode ray tube which have high vacuum inside.

These rays moved the paddled wheel when placed in their path.This means these rays have some momentum.

When the magnetic field was applied to the cathode rays they showed deflection. This means these cathode rays must have some charge because only charged particles can show deflection when place in magnetic field.

Later is was found by J.J Thomson that these are negatively charged.

How ?

We Know that like charges repel each other and unlike charges attract each other. This means if a particle has negative  charge then it will attract the positive charge.

J.J Thomson studied these cathode ray tubes and found that these rays repels the negative charge particles .Hence these rays must be negatively charged.

He called these tiny Particle "electron"