To create a rectangular mosaic where the length is 2 centimeters longer than the width (w), we can denote the width of the mosaic as \( w \) and the length as \( w + 2 \). The area of the rectangle (which must equal the total area of the tiles) is given by
\[ \text{Area} = \text{length} \times \text{width} = (w + 2) \times w. \]
Since John has 48 square centimeters worth of tiles, we set the area equal to 48:
\[ w(w + 2) = 48. \]
This equation represents the area of the rectangle in terms of its width. Thus, the correct equation that John can solve to find the greatest width \( w \) is:
\[ w(w + 2) = 48. \]