What does A485 mean? And what does the values on the y-axis expresses?
$A_{485}$ means the same thing as the text of the multiple choice question "absorbance of the solution at 485 nm". 

"nm" is an abbreviation for nanometers.

In other words, the absorbance of light having a wavelength of 485 nanometers is plotted on the y-axis of the graphs.