What is the smallest unit of time?
(A) Second
(B) Minute
(C) Hour
(D) Year

Hint : Time in physics is operationally defined as “what a clock reads”. Time is one of the seven fundamental physical quantities in both the international system of units and international system of quantities.

Complete step by step solution:
The basic unit of time is the second.
Time is often referred to as the fourth dimension, along with three spatial dimensions. It is a component quantity of various measurements used to sequence events, to compare the duration of the events or the intervals between them, and to quantify rates of change of quantities in material reality or in the conscious experience.

Additional information:
The international unit of time, the second, is defined by measuring the electronic transition frequency of the calcium atoms. Time is also of significant social importance having economic value as well as personal value due to an awareness of the limited time in each day and in life spans. A large variety of devices have been invented to measure time. The study of these devices is called horology. The hourglass uses the flow of sand to measure the flow of time. They were used in navigation. In the 11th century, Chinese inventors and engineers invented the first mechanical clock driven by an escapement medium. The solar day is the time interval between two successive passages of the sun across the local meridian. The local meridian is an imaginary line that runs from the celestial North Pole to the celestial South Pole passing directly over the head of the observer. At the local meridian the sun reaches its highest point on its daily area across the sky.

Note:
Time is used to define other quantities such as velocity so defining time in the terms of such quantities would result in the circularity of the definition. The operational definition of time does not address why events can happen forward and backwards in space, whereas events only happen in the forward progress of time.