What is the derivative of y=e−1y=e^(-1)?

y in this case is a constant. The the derivative is nil, as y=e^(-1) is a straight horizontal line.

'y' is  1/e  ... about 0.3679 . It's not a function of anything, and it doesn't change
in response to a change in anything else.

'y' is what's known as a 'number' or a 'constant', and its derivative with respect
to anything is zero.
 
When graphed, the equation  [  y = e^-1 ] is a straight horizontal line.  Its
slope is zero, and it intercepts the y-xis at the point  (0, 0.3679...). (rounded).

What mathematical statement is missing ? ? the answer is b