Final answer:
The experience where you feel you have reached the limits of your memory while calculating a sum is best explained as the capacity limit of working memory.
Step-by-step explanation:
The experience you are describing, where you feel that you have reached the limits of your memory while calculating the final sum of 5 two-digit numbers, is best explained as the capacity limit of working memory (option b). Working memory is the part of our memory system that temporarily holds and processes information. It has a limited capacity and can only hold a certain amount of information at a time. In the case of adding up the numbers, the working memory may become overwhelmed when trying to hold and process all the numbers simultaneously.