What is the symbol for the basic unit of time?

The symbol for the basic unit of time in the International System of Units (SI) is represented by the letter 'S'. This symbol stands for "second," which is the fundamental unit used to measure time. The second is defined by the frequency of radiation corresponding to a specific electronic transition of the cesium-133 atom. It is the standard unit that is widely used across all scientific disciplines for measuring intervals of time, thereby making it a crucial aspect of timekeeping and time-related calculations.

The other options presented represent different base units in the SI system: 'M' typically stands for meter, which is the unit of length; 'Kg' represents kilogram, the unit of mass; and 'A' signifies ampere, which is the unit of electric current. Each of these units serves an important role in their respective domains, but none are related to the measurement of time, which is specifically denoted by 'S'.