I am permitted to answer only your first question in terms of eNotes rules. Please post them separately (individually). What I can do is explain how compound, complex and compound-complex sentences work which should help you. So to answer :
a) is a compound sentence.
At 11:40 p.m on April 14, lookouts in the ship's crow's nest saw an iceberg directly in the vessel's path, but it was too late to change course.
Read the sentence from the beginning and stop at ...path. Do you notice that that is actually a complete sentence on its own. It can stand independently and would be a simple sentence as it has only one verb (saw).
The 'but' extends the sentence and this part of the sentence also has its own verb (was). 'But' is what we call a co-ordinating conjunction and it joins what would otherwise be two individual sentences. If we left 'but' out of the sentence "It was too late to change course" can also stand alone.
This then is a COMPOUND sentence.
Compound sentences always use co-ordinating conjunctions. Remember the word FANBOYS - each of these letters in 'fanboys' represents the first letter of a co-ordinating conjunction
For, and, nor, but, or, yet, so
For COMPLEX sentences, we use subordinate conjunctions - there are a lot so I will give you a few examples:
because, after, if, according to, although, when, etc.