The Langmuir adsorption isotherm is deduced using  the assumption: 

• the adsorption takes place in multilayers

• The adsorption sites are equivalent in their ability to adsorb the particles

• The adsorbed molecules interact with each other

• The adsorbed molecules interact with each other

A.

the adsorption takes place in multilayers

The main points  of Langmuir theroy of adsorption are as:
i) Adsorption takes place on the surface of the solid only till the whole of the surface is completely covered with a unimolecular layer of the adsorbed gas.
ii) Adsorption consist fo two opposing processes (a) condensation, and (b) evaporation.
iii) The rate of condensation depend upon  the uncovered surface of the adsorbent available for condensation.