To start sorting after the nth column of the (m+l)th field, the option to be used with the sort command is m.n.
Option A) -m.n - This option is incorrect because the - symbol is used to reverse the sorting order, not to specify the starting column.
Option B) m.n - This option is correct because it specifies that the sorting should start after the nth column of the (m+l)th field.
Option C) n.m 1 - This option is incorrect because the order of the values is reversed. The correct format is m.n.
Option D) (m l).n - This option is incorrect because it uses parentheses, which are not required in the syntax of the sort command.
Therefore, the correct answer is option B) m.n. This option is correct because it specifies the correct syntax to start sorting after the nth column of the (m+l)th field.