Basic standard error and Confidence errors

Hi all,
I’m currently taking a statistics class and very much regretting it as I find in class I understand the concepts initially yet can never seem to apply them or get confused and thus struggle with the basics.
I would very much appreciate help with these homework questions
For all 4 questions I would assume graph b will be significantly different as the variations is mean is large and there is no overlap between the error bars at all?
For part a and b: I think the answer is graph a and b
I'm not sure what difference it makes in part c and d to change from two standard errors to one and then to standard deviation ( I thought standard error was just another way or representing standard deviation for the whole population).

Any help would be really appreciated!