A clock is showing time at 4 o'clock, what will be the time after the hour hand rotates 90 degrees.

Given: A clock is showing time at 4 o'clock.

A clock has 360 degree, and it is divided into 12 hours.

​$$\frac {360}{12}$$ = 30 degrees

So, for every 90 degree rotation:

​$$\frac {90}{30}$$ = 3 hours

The initial time is 4 o’clock.

Adding 3 hours:​

4 + 3 = 7 o' clock

The time after the hour hand rotates 90 degrees is 7 o' clock.​

Thus, correct option is (b).

​