An exciton is: a charged particle made of electrons a charged particle made of holes a neutral particle made of an electron and hole a photon in an excited state

A neutral particle made of an electron and hole

Explanation:

Exciton

It is the combination of an electron and a hole ( hole refers to the vacancy of an electron ) . And , as both the electron and the hole have the same charge but the polarity is opposite , the combination will lead to a neutral compound , i.e. ,  Exciton have no charge and so neutral .

It is free to move in the nonmetallic crystal and since it charge less , it is difficult to detect it directly .

b. acid is the correct answer @jayxboof all social media

acid : acids are those which accepts an electron-pairs.

base : bases are those which donates ab electron-pairs.

alkali : it is a base that dissolves in water. it is basically a base and the ph greater than 7.

hydroxide ion : it is a poly-atomic ion which is negatively charged.