Final answer:
The radius of the circle tangent to the y-axis with a center at (-5,1) is the absolute value of the x-coordinate of the center, which is 5 units. The correct answer is A. 5.
Step-by-step explanation:
The question is asking for the radius of a circle that is tangent to the y-axis with a center at (-5,1) in the standard (x,y) coordinate plane. To find the radius of the circle, we can look at the horizontal distance from the center of the circle to the point of tangency on the y-axis.
Since the x-coordinate of the center is -5, and tangency to the y-axis would occur at x = 0, the radius of the circle is simply the absolute value of the x-coordinate of the center, which is the distance from (-5,1) to (0,1). That distance is |-5 - 0| = 5. Hence, the radius of the circle is 5 units.
Therefore, the correct answer is: A. 5