Universal time is based on
A. Rotational effect of the earth about its axis
B. Vibration of cesium atom
C. Orbit motion of the earth around the sun
D. Oscillation of quartz crystal

Hint: Use the concept of universal time measurement system.
Use the concept of rotational effect on the earth about its axis.
Use the concept of the orbit motion of the earth around the sun.

Complete step by step solution:
Earth rotation: Imagine a line passing through the Centre of the earth that goes through both the north and South Pole. This imaginary line is called the axis.
Earth spins around its axis, tested as a top spin around its spindle. This spinning movement is called rotation. An observer in space will see that earth requires 23 hours 56 minutes and 4 seconds to make one complete rotation on its axis.
Universal time is the time which depend upon the rotation of the earth and time can be dew berate by the observing celestial bodies crossing the noon time every day astronomers found that it was more pointer to establish time by looking at the stars as they cross the midday instead of observing the position at the sun on the sky. Monitoring of earth and universal time rotation. International earth rotation and reference system.

Note: Students should have an idea about the earth rotation.
Students should have ideas about the different time measurement systems.
Students should have an idea of vibration cesium atoms.