The correct answer is A. saying.
The other options are B. telling, C. talking, and D. discussing.
B. telling is incorrect because it implies that the listener knows what the speaker is talking about, but they are simply not paying attention.
C. talking is incorrect because it implies that the listener is aware of the topic of the conversation, but they are not interested in it.
D. discussing is incorrect because it implies that the listener is actively participating in the conversation, but they are not understanding what the speaker is saying.
The word “saying” is the most accurate word to use in this context because it implies that the listener is not understanding the speaker, regardless of whether they are paying attention, interested, or participating in the conversation.