What instrument can be used to check that constant attitude is maintained?
Multiple choice questions are rarely about picking the only correct answer but instead often challenge you to pick the most correct answer out of a choice of 2 or more potentially correct answers.
I think that is the case here. While the VSI may well indicate to you that you are climbing or descending, it is unreliable for monitoring as it will be affected by small fluctuations such as up/downdrafts and lags quite a bit behind actual control input.
The altimeter, on the other hand, is quite stable and does not fluctuate quite so readily. If you're scanning and over a period of a minute your altitude has increased by 100 ft you can be pretty sure you're climbing - in that time the VSI may have indicated a climb, a descent and another climb etc. If you had scanned it while in a short-lived descent you might be fooled into thinking the long term affect was a descent. It was not - you were climbing overall.