Which time format includes an optional fractional part?

• A. HH:MM:SS
• B. HH:MM:SS[.sf]
• C. HH:MM
• D. HH:MM:SS[.f]

Answer: (B)

The concept being tested is that a time format can include an optional fractional second component, written after the seconds and indicated as optional by brackets. In this format, you have a base of hours:minutes:seconds, and you may append a decimal point followed by one or more digits representing the fraction of a second. The brackets show this portion can be omitted or included as needed, and the digits can vary in length.

This format is best because it explicitly marks the fractional part as optional and allows multiple digits for precision (for example, 12:34:56 or 12:34:56.1234). Other options either omit the fractional part entirely, fix the format to only whole seconds, or restrict the fractional portion to a single digit, which doesn’t accommodate longer fractional seconds.