Question

It has been an objective in metrology to define the kilogram in terms of fundamental constant, in the same way that the ________ is now defined in terms of speed of light.
(A) Centimetre
(B) Metre
(C) Decimetre
(D) None of these

Answer 1

May 20, 2019, marks one of the biggest shake-ups in the history of measurement. The definitions of the seven base units of the SI (the International System of Units) are now completely different. Now, all the units are defined in a common way using what the BIPM calls the explicit constant formulation.

Complete step by step answer:
The SI base units are the standard units of measurement. It is defined by the International System of Units (SI). There are seven main SI base units. They are second for measurement of time, metre for measurement of length, the kilogram for measurement of mass, the ampere for measuring electric current, the kelvin for measuring temperature, the mole for measuring the amount of substance, and the candela for measuring luminous intensity.
We should write the names and symbols of SI base units in lowercase. There are some exceptions, the symbols which are named after a person are written with an initial capital letter.
For example, the kilogram has the symbol $kg$, but the kelvin has symbol $K$, because it is named after Lord Kelvin.
In May 20, 2019 the national metrology institutes (NMIs) as well as the International Bureau of Weights and Measures (BIPM) defined new definitions for the seven base units.
1. The second, symbol $s$, is the unit of time. The magnitude of time is set by fixing the numerical value of the ground state hyperfine splitting frequency of the caesium $133atm$, at rest and at a temperature of $0K$, to be equal to exactly 9 192 631 770.
2. The metre, symbol m, is the unit of length. Its magnitude is set by fixing the numerical value of the speed of light in vacuum to be equal to exactly 299 792 458.
3. The candela, symbol $cd$, is the unit of luminous intensity in a given direction Its magnitude is set by fixing the numerical value of the luminous efficacy of monochromatic radiation of frequency, $540 \times {10^{12}}Hz$ to be equal to exactly 683.
4. The kilogram will continue to be the unit of mass, but its magnitude will be set by fixing the numerical value of the Planck constant to be equal to exactly $6.62606 \times {10^{ - 34}}.$
5. The ampere will continue to be the unit of electric current, but its magnitude will be set by fixing the numerical value of the elementary charge to be equal to exactly $1.60217 \times {10^{ - 19}}.$
6. The kelvin will continue to be the unit of thermodynamic temperature, but its magnitude will be set by fixing the numerical value of the Boltzmann constant to be equal to exactly $1.3806 \times {10^{ - 23}}.$
7. The mole will continue to be the unit of amount of substance of a specified elementary entity, which may be an atom, molecule, ion, electron, any other particle or a specified group of such particles, but its magnitude will be set by fixing the numerical value of the Avogadro constant to be equal to exactly $6.02214 \times {10^{23}}.$
From above we come to the conclusion,
The metre is now defined in terms of the speed of light.

Hence the correct answer is option (B) metre.

Note: A number of other units, such as the litre, electron volt and astronomical unit are not formally part of the SI, but are accepted for use with SI.