I guess you just have to, by adding some value to both sides, line up the equation so that it fits the circle form.

x² + y² - 8x + 2y = -1

(x² - 8x) + (y² + 2y) = -1

For x² - 8x to work as part of the result of squaring x - h, you need 16 to create x² - 8x + 16, which is (x - 4)².

For y² + 2y to work as part of the result of squaring y - k, you need 1 to create y² + 2y + 1, which is (y - (-1))².

So add 16 and 1 to both sides of the equation to get the following.

(x² - 8x + 16) + (y² + 2y + 1) = 16

So the radius is √16 = 4

The correct answer is D.