Which among the following is a cation?

The correct answer is option 1. Ammonium.

A cation is a positively charged ion formed when an atom loses one or more electrons. Ammonium, \(NH_4^+\), is a polyatomic ion formed by the donation of a lone pair of electrons from a nitrogen atom to a hydrogen ion (proton). This results in the formation of a positively charged ammonium ion. Therefore, ammonium is a cation.

Iodide (option 2), chloride (option 3), and fluoride (option 4) are all negatively charged ions known as anions. They are formed when atoms gain one or more electrons, resulting in a net negative charge.