The correct option is mol.
Explanation
The mole is the base unit in the International System of Units (SI) used to measure the amount of substance. It quantifies the number of elementary entities (such as atoms, molecules, or ions) present in a sample.
- mol is Correct: The official SI symbol for the unit mole is mol. It is used in equations and measurements to denote quantity (e.g., 5 mol of Oxygen).
- m is Incorrect: The symbol m represents the metre, which is the SI unit of length.
- M is Incorrect: The symbol M is conventionally used to denote Molarity (concentration in moles per litre) or Molar Mass, not the unit of amount of substance itself.
- mole is Incorrect: mole is the full name of the unit. The question specifically asks for the symbol, which is the abbreviated form used in scientific notation.
Key Takeaway: There is a strict distinction between a unit's name and its symbol. For the amount of substance, the name is mole, but the symbol is mol.