Which operation will determine the number of days in a year if there are 365 days in a regular year?

To determine the number of days in a year when there are 365 days in a regular year, the best operation to consider is multiplication. In a standard context, one year is defined as having 365 days. If the question was asking about a specific number of years, for example, to calculate the total number of days over multiple years, multiplication would be used. For instance, you would multiply the number of years by 365 days to find the total days across those years.

However, if only one year is in focus, stating that there are 365 days means this is a given fact rather than something to be calculated through an operation. The operation to determine the number of years in a specific number of days would involve division, but that's not the situation described here.

Understanding the context and setting of the problem is essential in determining the most appropriate operation.