The correct answer is option 'A': Coulomb.

The SI unit of electric charge is the Coulomb (C). It is named after the French physicist Charles-Augustin de Coulomb, who made significant contributions to the study of electromagnetism. The Coulomb is defined as the amount of electric charge transported by a constant current of one Ampere (A) in one second.

To understand why the Coulomb is the unit of electric charge, let's delve into the concept of electric charge itself.

Electric charge:
Electric charge is a fundamental property of matter. It is associated with the presence or absence of electrons in an atom. Electrons are negatively charged particles, while protons are positively charged particles. The charge of an electron is considered as the basic unit of charge, denoted as 'e', which is approximately equal to -1.6 x 10^-19 Coulombs.

SI unit of charge:
The SI unit of charge is defined based on the amount of charge carried by electrons or other charged particles. The Coulomb is used to measure the quantity of electric charge. It represents a large amount of charge, as a single Coulomb is equivalent to the charge of approximately 6.24 x 10^18 electrons.

Relation to current:
Current is the flow of electric charge in a circuit. It is measured in Amperes (A), where one Ampere is defined as one Coulomb of charge passing through a given point in one second. So, if a current of 1 Ampere flows through a wire for 1 second, it represents a transfer of 1 Coulomb of charge.

Practical applications:
The Coulomb is used to measure various quantities related to electric charge, such as the charge on capacitors, the charge of particles in particle accelerators, and the charge stored in batteries. It is also used in calculations involving electric fields, electric potential, and electrical energy.

Summary:
The SI unit of electric charge is the Coulomb (C). It is defined as the amount of charge transported by a current of one Ampere in one second. The Coulomb is used to measure the quantity of electric charge in various practical applications.