As written, your function should not return the given list; it should return the value of A[0] * k, the scalar 50.
Let's walk through the code for a moment, as you should have done with a print
statement before posting.
for i in A:
# i is a local variable; it takes on the value of A[0], or 5
i = i*k
# i is now 50
return i
# You return 50 to the calling program -- which ignores the return value.
At this point, the function is done, gone, exited, and will execute no more until you call it again.
To get the list (you have a list, not an **array*) altered, you can change each element in place. Don't leave the function until you've processed all of the elements.
for i in range(len(A)):
A[i] *= k
return