Name and define the S.I. unit of time. How is it related to (i) minute (ii) hour, (iii) day and (iv) year?

The S.I. unit of time is second (s). One second is defined as $\dfrac{1}{86400}$ part of a mean solar day.

(i) 1 minute = 60 seconds

(ii) 1 hour = 3600 seconds

(iii) 1 day = 86400 seconds

(iv) 1 year = 3.15 x 10⁷ seconds