The correct option is C The adsorption sites are equivalent in their ability to adsorb the particles.
If you remember, we have talked about 3 assumptions that Langmuir made to derive his theory of adsorption.
They are :
1. Every adsorption site is equivalent
2. The ability of a particle to bind on the site is independent of whether or not nearby sites are occupied.
3. The adsorption forms only a uni-molecular layer
We can see that option c is one of the assumptions and the others are not.