Final answer:
The function y = -5 is represented by a horizontal line at y-intercept -5, and thus, the graph is a straight line but not a linear function as it lacks an x-component. The closest correct option provided is C: 'It is not a linear function, but its graph is a straight line.'
Step-by-step explanation:
The student has provided the function y = -5 and is asking which of the provided statements correctly describes the function's graph. A function in the form of y = a + bx represents a linear equation, where 'a' is the y-intercept and 'b' is the slope. If 'b' is greater than 0, the slope is positive, which means the line slopes upward to the right. If 'b' is less than 0, the line has a negative slope, sloping downward to the right. If 'b' equals 0, the result is a horizontal line.
In the given function, there is no 'x' term, meaning 'b' is 0. This results in a horizontal line at the y-intercept of -5. As there is no slope and the line is parallel to the x-axis, the statement that correctly describes the function would be: "It is not a linear function, but its graph is a straight line." In the context of the given options, none of the provided options match exactly, but option C ("It is not a linear function, but its graph is a straight line.") is the closest in description if we consider a linear function to require both an 'x' and 'y' component.
Thus, the final answer to the student's question is option C.