Answer: The word that would correctly complete this sentence is "older".
Explanation: The sentence provided includes a comparative structure, that is to say that it establishes a difference between two objects by means of comparison. A comparative structure always includes a comparative adjective and it never presents a superlative adjective. For that reason, "oldest", which is a superlative adjective, cannot complete this sentence. In that way, the comparative form of "old", which is "older" and not "more older" because "old" is a one-syllable adjective, is the word that fills in the blank.