7. What is the value of the postfix expression 6 3 2 4 + – *?a) 74b) -18c) 22d) 40

The postfix expression is evaluated using a stack. The steps are as follows:

1. Push 6 onto the stack.
2. Push 3 onto the stack.
3. Push 2 onto the stack.
4. Push 4 onto the stack.
5. Encounter '+', so pop 2 and 4 from the stack, add them to get 6, and push 6 onto the stack.
6. Encounter '–', so pop 6 and 3 from the stack, subtract 6 from 3 to get -3, and push -3 onto the stack.
7. Encounter '*', so pop -3 and 6 from the stack, multiply them to get -18, and push -18 onto the stack.
8. At the end of the expression, the stack contains the final result, which is -18.

So, the answer is (b) -18.