One second is defined to be equal to:
A) 1650763.73 time periods of Kr clock
B) 652189.63 time periods of Kr clock
C) 1650763.73 time periods of Cs clock
D) 9192631770 time periods of Cs clock

Hint: Second is the standard unit of time. The most commonly used element for atomic clocks is the Cesium 133 atom. Its transition or resonant frequencies are very much reliable and accurate. Hence, it is used to define a second.

Complete answer:
The unit of second was initially defined based on the Earth's rotation; the Earth rotates on its axis once per day, with a second being defined as \[{{\dfrac{1}{86400}}^{th}}\] part of a (mean solar) day. But, due to irregularities in the frequency of earth’s rotation, using the Earth's rotation as a definition for time is not practical. Hence, Cesium-133 is used for defining one second because of the reliable frequency of microwave radiations it emits. The light or radiant energy, in the form of microwaves, is emitted at a very precise frequency from the atomic transition of cesium 133 atom. Upon observing exactly 9,192,631,770 cycles or periods of this electromagnetic wave, a second is said to have passed. Hence,
\[\text{1 second = 9192631770 time periods of Cs clock}\]

Therefore, the answer is option D.

Note:
There are many practical uses for extremely accurate clocks. The most important is in Earth's Global Positioning System (GPS). It requires atomic clocks to provide accurate measurement of distances. These clocks also provide reliable time reference for scientific research applications, such as measuring the variations in the frequencies of pulsars.