which of these functions is linear?

The only linear function is:
y = x - 5
So the correct option is B.
A linear function is a polynomial of degree 1, and can be written as:
y = a*x + b
So we have a coefficient multiplying the variable and a constant term, nothing else.
By comparing this with the options, we can see that the only linear function given is the one in option B.
y = x - 5
Where a = 1, and b = 5.
If you want to learn more about linear functions, you can read:
https://brainly.com/question/4025726