Concept:The correct phrasal verb is one that means to understand, hear, or see something with difficulty.
Explanation:The sentence says: I just couldn't .......... what he was saying.
Here, the speaker refers to difficulty in following or understanding what the speaker was saying.
"Make out" means to hear, see, read, or understand something, especially when it is not clear.
Therefore, the correct choice is "make out."
The other options are incorrect: "make do" means to manage with something, "make up" means to invent or reconcile, and "make by" is not a standard phrasal verb.
Answer:B. make out