Who is most likely the "old man" that Lady Macbeth mentions?
Macbeth
the doctor
Duncan
Banquo

Banquo.

Explanation:

William Shakespeare's Macbeth tells the tragic story of how the protagonist Macbeth brought his own ruin by being over-ambitious. He, along with his wife Lady Macbeth, used force and even illegal and traitorous deeds to get their wishes to come true.

In Act V scene i, the doctor was summoned by the gentlewoman who had reported that her mistress has been sleepwalking for some nights now. Once when both of them observed Lady Macbeth sleepwalk again, they heard her say "who would have thought the old man to have had so much blood in him." This "old man" that she mentioned is Banquo, whom she had just murdered so that the three witches' prophecy about her husband might come true.  

faulty parallelism is a construction in which two or more parts of a sentence are equivalent in meaning but not grammatically similar in form.

the correct version of this faulty parallelism in the sentence is:

the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.

the problem is the last sentence, the first two have a basic subject+verb+object structure.

so, to give a proper balance to the structure, or better said, to give a parallel structure, we must write a basic s+v+o structure in all three sentences:

the hikers knew that the hike was six miles, that it usually required the entire day, and it would not be wise to start after 10: 00 a.m.