Which is not an effective organizing principle for the study of history

I believe that the question comes with the following options:

a. eras

b. major time periods

c. topics

d. wars

e. year-by-year accounts

out of those, the worst option is "wars" - It's not the only ineffective organizing principle, but it's an example for a bad organizing principle. This is because wars don't happen regularly and there are longer periods of time between wars - so organizing the study of history according to wars would skip those long periods. So, generally anything that is not a regular occurence would be a bad organizing principle for the study of history.