Answer:
Reasonable means that it follows some type of logic, that may or may not be the correct logic path to reach a given answer.
For example, if we want to calculate the difference between 23091 and 12, we may expect to have a positive number that is near 23091, so the answer 23080 may seem reasonable, but it is actually incorrect, the real answer is 23079
Now, this is a particular example, but you may find a lot of cases where an answer may seem reasonable but is not actually correct, especially when you work with things like quantum physics or special relativity, where the tings do not work as we accustomed, so the things that "seem reasonable" may not be.
