在C语言中正确使用if/else语句的问题。

I am still very new to programming, so I am not sure the correct action to take. I can't seem to get the program to display a different option when selected by the user. Either the first option displays, or it shows the 'Invalid Entry' text. I am only going to include the problem code, since I've tested the rest without the if/else statements and it calculates and displays correctly.I am still very new to programming, so I am not