Which of the following is the first particle discovered which is believed to be elementary, that is, not made up of further constituents?

Correct option is C

Sol. The electron was discovered by J.J. Thomson in 1897, and it was the first particle believed to be elementary (i.e., not made up of smaller particles). It has a negative charge and is one of the fundamental building blocks of matter.
Explanation of each option:
· (a) Glucon – Incorrect. Glucon is not a particle; it's related to glucose (a carbohydrate), and has nothing to do with atomic structure.
· (b) Photon – Incorrect. A photon is also elementary, but it was discovered after the electron and is associated with light.
· (c) Electron – Correct. First discovered elementary particle, fundamental in electricity, chemistry, and atomic structure.
· (d) Proton – Incorrect. The proton was discovered later (in 1917) by Rutherford and is not the first known particle.