Which of the following is not correct?
$(i)$. The basic unit of time is second.
$(ii)$. Every object moves with a constant speed.
$(iii)$. Distances between two cities are measured in kilometres.
$(iv)$. The time period of a given pendulum is constant.
$(v)$. The speed of a train is expressed in m/h.

$(i)$. The basic unit of time is second. -Correct

MKS System is followed internationally for measuring length, mass, and time. MKS stands for-

$M\rightarrow$ Metre for measuring the length of a physical quantity.

$K\rightarrow$ Kilogram for measuring the mass of a physical quantity.

$S\rightarrow$ Second for measuring time.

$(ii)$. Every object moves at a constant speed. -Not correct

Different objects can travel at different speeds. An object may move at a constant speed and may not move at a constant speed. It depends upon the situation.

$(iii)$. Distances between two cities are measured in kilometers. -Correct

The distance between two cities is very long length. So, the distance between two cities is measured and expressed in kilometers.

1 kilometer=1000 meter

$(iv)$. The time period of a given pendulum is constant. - Not Correct

The time period of a pendulum never remains constant.

$(v)$. The speed of a train is expressed in m/h. -Not correct

A train moves with a greater speed which is difficult to measure and express in m/h as it travels very long distances. Either the speed of the train is expressed in km/h or in m/s.