The correct answer is: A. 1 hour.
A unit hydrograph is a hydrograph that represents the runoff from a unit storm of a specified duration and intensity. The unit duration is the length of time over which the storm is assumed to occur. The unit intensity is the rate of rainfall per unit area that is required to produce a unit hydrograph.
The best unit duration of storm for a unit hydrograph is 1 hour. This is because most storms have a duration of less than 1 hour, and the unit hydrograph can be used to represent the runoff from these storms with a reasonable degree of accuracy.
The other options are not as good choices for the unit duration of storm for a unit hydrograph. Option B, one-fourth of basin lag, is too short a duration. This is because most storms have a duration that is greater than one-fourth of the basin lag. Option C, one-half of basin lag, is also too short a duration. Option D, equal to basin lag, is too long a duration. This is because most storms have a duration that is less than the basin lag.
In conclusion, the best unit duration of storm for a unit hydrograph is 1 hour.