Which correctly punctuates an introductory element? a: yes, you should take the dog out for a walk every afternoon. b: having, finished dinner at eight, she was ready server dessert by nine thirty. c: by daybreak everything has settled down. d: in the morning after we eat breakfast you should apologize.

I would say the the correct answer is B.

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.