Question

Which of the following is not the unit of time?
A.) Hour
B.) Nano second
C.) $ \mu $ second
D.) Light year

Answer 1

Hint: The unit of a physical quantity is an arbitrarily chosen standard which is widely accepted by the society and in terms of which other quantities of similar nature may be measured.
A unit of time is any particular time interval, used as a standard way of measuring or expressing duration.
In this question, we will check which of the given options is not representing the unit of time.

Complete Step by step complete answer
SI unit of time is second. Other important units of time are minute, hour, day, week, month and year.
So, 1 Nano second = $ {{10}^{-9}} $ second
1 microsecond = $ {{10}^{-6}} $
Thus, options A, B, C are units of time.
On the other hand, light year is the unit used to describe astronomical distances. It is defined as the distance travelled by light in one earth year.
Thus, option D is the correct answer.

Additional information
Definition of SI unit of time i.e. second: -One second is defined as “9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the Cesium-133 atom”.

Note
Remember the following details about the units of time.
1 minute = 60 seconds
1 hour = 60 minutes
1 day = 24 hours
1 week = 7 days
1 month = 28-31 days
1 year = 365 days (366 days in case of leap year
Also note that the light year is used to measure the distance of astronomical objects and one light year is equivalent to about $ 9\cdot 46 $ trillion kilometres (or $ 5\cdot 88 $ trillion miles).