Which of the following abbreviations requires to use of a period or periods

The answer to this question would be this: am (the abbreviation for before noon). This abbreviation is correctly written as A. M. or a. m. which means ante meridiem, or 'before noon'. This is the only abbreviation, among the other options given, that requires the use of a period or periods. For example, 10 A. M. Without the period, it would have a different and vague meaning.