'A' is correct. There are numerous grammatical problems with the other options, but it seems to me that the 4 options were meant to represent just 2 key grammar choices.
Firstly, there is a difference in the comparative term used. A and B use 'smaller' while C and D use 'smallest'.
'Smaller' is used when comparing the size of 2 things. 'Smallest' is the superlative, which means it is the 'most small of all', so best used when comparing many things. This rules out options C and D. To be honest, some native English speakers may well use 'smallest' in any comparison, but you've asked why AI told you that one option is right, and that's the obvious reason.
Secondly, the correct answer has to fit into the space after the partially written sentence. Option A completes the sentence as it is a relative clause (using "of which") that provides additional information about the subject and helps differentiate the two rooms based on their sizes. Option B is not constructed this way and it does not fit into the blank space after a comma. It is a clause in its own right so you would need a conjunction instead of the comma, ie "and". Using a comma as a conjunction is incorrect and referred to as a 'comma splice'.