Final answer:
Placing a red filter in front of the light source would result in the ball appearing red, because the filter would only allow red light to pass through and be reflected to the observer.
Step-by-step explanation:
If a red filter is placed in front of the light, the color of the ball would only allow red light to pass through. As a consequence, the ball would appear red because all other colors of light would be absorbed by the filter and only red light would be reflected off the ball to your eyes. This is due to the fact that a filter selectively transmits light of its own color while absorbing the other colors in the spectrum. Therefore, options b), c), and d) are incorrect since a red filter would not make the ball appear blue or green, nor would the ball's color remain the same.