Your equation would be y = 210x + 240
Each time you add a night, the price will go up by 210 so each night will have a common difference of 210. This eliminates options C and D.
Going back to the phrase I used "common difference" not "ratio," this tells you to use an arithmetic sequence.
Your answer should be option A if I'm not mistaken (haven't done arithmetic and geometric sequences in quite some time).