I can understand your confusion.
The 2 factors for rate of condensation to take place are:
- Temperature of the water vapour. The lower the temperature of the vapour, the faster it can condense because less energy is required to turn it from a gas to a liquid.
- The amount of water vapour in the air. The higher the concentration, the more gas particle will collide with the cold glass. You also need to know that the warmer the air, the more water vapour it can hold, ie. the concentration of water vapour is higher.
Notice the first one is about how fast the water vapour are converted based on the temperature and the second one is how much of water vapour is converted based on concentration and both points to rate of condensation.
In the above cup A and cup B example, we cannot see the how fast from the picture(and we were told they were exposed for the same amount of time) but we can see the how much, and given the information is a warmer place versus a colder place for the same amount of time, point 2 applies. Hence (c) can be an explanation for the phenomenon observed.