Final answer:
Text-to-speech software would not be useful for Rex when reading on a computer if he aims to visually read, since it does not directly aid in reducing visual strain.
Step-by-step explanation:
Rex has residual vision but fatigues easily. Among the options given to improve his reading experience on a computer, text-to-speech software would not be useful if he is specifically looking to visually read text on the screen. Large font size, high contrast display, and adjustable screen brightness are visual aids that can help reduce eye strain and assist with reading, by making text more legible and reducing glare. However, text-to-speech software converts text into audible speech, allowing a user to listen instead of read, which might be beneficial for someone with visual fatigue but does not directly assist with the act of reading visually.