Water is initially present in a state where it’s molecules are far apart. During a change of state ,it’s molecules slow down. Which change of state has most likely taken place?

Gas ⟶ liquid (condensation)  

Explanation:

If the molecules are far apart, they are in the gas phase.

As the temperature drops, the molecules lose kinetic energy.

They can no longer escape the attractive forces between them and their neighbours, so they stick together and form a liquid.

The most likely change of state is gas ⟶ liquid (condensation).

a gas, having a molecular weight of 13 kg/kmol and a kinematic viscosity of 0.25 cm2/s, flows through a pipe 0.25 m internal diameter and 5 km long at the rate of 0.4 m3/s and is delivered at atmospheric pressure.