Answer: 4 is to 2 
Explanation:
This is almost like a mind tricking question. 
The question asked number of tetras to catfishes. 
so you only have to focus on that. Not other unnecessary details
They gave you, 4 tetras, 9 corydoras, and 2 synodontis catfishes. 
So the ration would be 4 tetras to 2 catfishes.