Does the interest rate increase at a constant rate? 6 months = 2.4% 9 months = 2.9% 12 months = 3.0% 18 months = 3.1%

No.

From 6 mo to 12 mo, it increases 0.6%.
From 12 mo to 18 mo, it increases 0.1%. The change over a 6 mo period gets smaller as the period gets longer or the rate gets higher.

Neverminding for a second that the rate is in percents.

it seems what's being asked is, "is the change from a month to the next, constant", as in an arithmetic sequence, well, check the picture below, those coordinates do not give us a straight line, and therefore, we don't have an arithmetic sequence, and therefore, we don't have a "common difference".

and since we don't have a common difference, the rate is not constant.