Final answer:
The keyboard of a TTY is the component responsible for receiving characters in uppercase and transmitting in lowercase. Other components like the modem, printer, and monitor are not responsible for changing the case of letters.
Step-by-step explanation:
The component of a TTY (teletypewriter) that receives characters printed in all capital letters and transmits characters in lowercase letters is the keyboard. When a person types on the TTY keyboard, it converts the typed characters into electrical signals to be sent over a telephone line. Historically, TTYs were designed to communicate by sending lowercase characters, and they would receive messages in uppercase to distinguish between the two. This was particularly important for users who are deaf or hard of hearing, as TTYs facilitated their communication before the widespread adoption of modern text and video relay services.
The modem is involved in the modulation and demodulation of signals, but is not responsible for the case of letters. The printer is an output device that provides a paper copy of the text, while the monitor displays the information visually on the screen. Both of these components display text as it is received, without changing the case of letters.