SI unit for time is:
A) Minutes
B) Hours
C) Seconds
D) Days

Hint: SI stands for Systeme International i.e. International system of units is also known as metric units. SI units are basically the units of the physical quantity which are used and recognized all over the world. Any constant is determined by substituting values in its SI unit. There are seven basic SI units, one of which is for the time which we will discuss here.

Complete step by step answer:
Basically time can be measured in seconds or minutes or hours or days. All of these are units of time. But SI units of time are seconds that is the reason most of the constants are measured per unit second or for unit seconds. For example, the unit of speed of light is measured is m/s i.e. meter per seconds and other universal constants like acceleration due to gravity g, Planck constant h, universal gravitational constant G etc. are measured per unit seconds$^2$ or per second or for unit seconds.

Additional information:
Short period of time can be measured in microsecond or in nanosecond which has order of magnitude -6 and -9 respectively.
Other SI units are meters for length, kilograms for mass, ampere for current, mole for amount of substance, kelvin for temperature and candela for luminous intensity.

Note:
Half-life of radioactive elements are measured in days or even years, similarly the age of a person or planets or an object is measured in years. But unit light-years is not the unit for time it is the unit of distance which is covered by light in one years it is used in measured far distance objects in space.