Explanation:

- Option 1: the smaller of which

- Refers to one of two small rooms.

- Correctly uses "which" to refer back to the rooms.

- This is the correct choice.

- Option 2: the smallest of which

- Implies comparison among more than two objects.

- "Smallest" is not correct since there are only two rooms.

- Option 3: the smaller of them

- "Of them" is less precise than "of which."

- Grammatically less preferred.

- Option 4: smallest of that

- Incorrect structure.

- "That" does not clearly refer back to the rooms.