Shell variable expansion is a fundamental scripting concept included in Linux+ V8 objectives. In Bash and similar shells, variables are expanded only when they are interpreted within double quotes or unquoted contexts, and sometimes explicit syntax is required to avoid ambiguity.
The correct notation is ${num}, as shown in option D. Using curly braces around the variable name ensures the shell correctly identifies the variable boundary, especially when it is adjacent to other characters. This guarantees proper expansion of the variable's value.
The other options are incorrect. Single quotes prevent variable expansion entirely. The $(...) syntax is used for command substitution, not variable expansion. Quoting the variable name itself also prevents expansion.
Linux+ V8 documentation emphasizes ${VAR} notation as a best practice in shell scripting for clarity and correctness. Therefore, the correct answer is D.