So, there are 4 solutions.
Choice A is the only one that is changing at a constant rate. The reason is that for all three other choices the new rate is based on a different amount after the percent has been applied or the doubling of the ants has been applied. The price per day for the lunch Is constant because whatever the number of days is the amount would always stay the same for each of those days.
I believe it was a rectangle I'm not sure test it out.