So, you have 40 cats rescued in 5 weeks, right? Your first number is 40, second is 5, so you do 40/5==8, or 8:1 if you need a ratio (since you're going to have 8 cats per 1 week). Just from that first part, you can pick A. Then you have that 24:3 nonsense at the end of your answer choice; multiply your 8:1 by numbers until you get your 24:3 to see if it matches up.
2(8:1)=16:2
3(8:1)=24:3
4(8:1)=32:4
............
and you keep going. You're keeping your same ratio because if you divide your first number by your second number, you're still gonna get 8:1 (8 cats 1 week)