Consider the given expression,
[tex]56-21[/tex]The expression can be transformed as,
[tex](7\times8)-(7\times3)[/tex]Since 7 is a common factor, take it as common,
[tex]7(8-3)[/tex]Thus, the expression is obtained that,
[tex]56-21=7(8-3)[/tex]Therefore, option (B) is the correct choice.