Final answer:
To improve the readability of characters 'RN', adjusting the tracking is the correct solution. This will alter the spacing between letters, enhancing clarity and preventing characters from appearing merged.
Step-by-step explanation:
To correct the readability of the characters 'RN', you would adjust the tracking. Tracking involves adjusting the spacing between a range of letters to affect the overall density and readability of the text. This is particularly useful when two characters appear to be merged or are hard to distinguish, as might be the case with 'RN' where the right leg of the 'R' could blend with the 'N', depending on the font.
If the issue with 'RN' was that the characters were too close to each other, increasing the tracking would space them out more evenly, making them clearer to read. In contrast, font size (a) would simply increase the size of the characters, leading (b) would adjust the space between lines of text, and raster effects (d) would apply pixel-based effects that might not necessarily improve character readability.