Explanation:

- Option 1: Gap - This is not a function in Excel for calculating the difference in days between two dates.

- Option 2: Dates - This option does not correspond to a valid Excel function for calculating days between dates.

- Option 3: Days - This function indeed calculates the number of days between two dates. It is used as `DAYS(end_date, start_date)`.

- Option 4: Days 360 - This calculates the number of days between two dates based on a 360-day year (12 months of 30 days each). Suitable for financial calculations.

- Option 5: SPAN - This is not a standard Excel function related to date calculations.