57.  What is the value of the postfix expression 6 3 2 4 + – *?(a) 1(b) 40(c) 74(d) -18

Sure, I can help with that. The postfix expression is evaluated using a stack. Here are the steps:

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 together to get 6, and push 6 back onto the stack.
6. Encounter '–', so pop 6 and 3 from the stack, subtract 6 from 3 to get -3, and push -3 back onto the stack.
7. Encounter '*', so pop -3 and 6 from the stack, multiply them together to get -18, and push -18 back onto the stack.
8. Now, there are no more elements in the expression, so the value of the expression is the only element left on the stack, which is -18.

So, the answer is (d) -18.