These chairs are more comfortable ... the old chairs. From. Than. Then. As.

The correct answer is Than.

Explanation:

In this sentence, we are comparing the comfort level of two sets of chairs: 'these chairs' and 'the old chairs'. When making a comparison in English, we use 'than' after a comparative adjective (in this case, 'more comfortable').

The complete sentence is: 'These chairs are more comfortable than the old chairs.'