The way that real-time text or RTT on her phone can help her is that;
C: On phone calls, it will convert speech into text
We are told that Deirdre has a hearing disability. This means that she has difficulty in hearing words clearly but can read.
Thus, from her friends advice about real time text or RTT, it will aid her because whenever someone calls her on the phone, it can convert the content of the call speech into text.
The missing options are;
A. It will make her phone vibrate to notify her when she gets a phone call
B. It will make her phone vibrate to notify her when she gets a text message.
C. On phone calls, it will convert speech into text. D. While text messaging, it will convert text into speech
Read more on speech to text at; https://brainly.com/question/2375490
