In which year the SI system of units was developed and recommended by the general conference on weights and measures?
A) \[1951\]
B) \[1961\]
C) \[1971\]
D) \[1981\]

Hint:SI unit is an abbreviation of the French word System International. The International System of Units (SI) is the metric system that is universally accepted as a standard for measurements.

Complete step by step solution:
The SI system of units was developed and recommended by the General Conference on Weights and Measures in\[1971\].
SI units have an important contribution in scientific and technological research and development. It comprises \[7\] base units which are used for defining \[22\] derived units.
The basic \[7\] fundamental units for fundamental quantities are as following,
Physical Quantities
S.I. Unit
For Mass is Kilogram (\[kg\])
For Length is Meter (\[m\])
For Time is Second (\[s\])
For Electric current is Ampere (\[A\])
For Temperature is Kelvin (\[K\])
For Amount of substance is Mole (\[mol\])
For Luminous intensity is Candela (\[cd\])

Unit of length, meter (\[m\]): Meter is the SI unit of length and is defined by taking the fixed value of the speed of light in vacuum. It is expressed as \[m{{s}^{-1}}\].

Unit of mass, kilogram (\[kg\]): Kilogram is the SI unit of mass and is defined by taking the fixed value of the Planck constant. It is expressed as \[kg{{m}^{2}}.{{s}^{-1}}\].

Unit of time, second (\[s\]): Second is the SI unit of time and is defined by taking the fixed value of Cesium frequency. It is expressed as \[{{s}^{1}}\].

Unit of electric current, ampere (\[A\]): Ampere is the SI unit of electric current and is defined by taking the fixed value of the elementary charge.

Unit of thermodynamic temperature, Kelvin (\[K\]): Kelvin is the SI unit of thermodynamic temperature and is defined by taking the fixed value of Boltzmann constant \[k=1.380649\times {{10}^{-23}}\].

Unit of the amount of substance, mole (\[mol\]): Mole is the SI unit of amount of substance and is defined by the fixed value of Avogadro constant \[{{N}_{A}}\]. One mole contains \[6.02214076\times {{10}^{23}}\] elementary entities and is expressed as \[mo{{l}^{-1}}\].

Unit of luminous intensity, candela (\[cd\]): Candela is the SI unit of luminous intensity and is defined by the fixed value of the luminous efficacy.

Note: The fundamental units are independent of any other units including themselves.
There are a total of seven fundamental units in the SI unit system.
The derived units are those units which are obtained by mathematical operations of fundamental units.