将命令行参数转换为char

I'm working with a command line arguments for the first time and I need to convert one of the string arguments to a character for testing. I tried testing it as a string and it wasn't working. In the program the user needs to enter an e or E or d or D (sample input at top of code). I haven't done much with conversions in C either so any help would be great. Thanks.I'm working with a command line arguments for t