I think the questions that must be ask are:
a.What does “encryption” mean?
d.What are “key distribution problems”?
All the text is based on the concept "encryption" so it is very important to know what it means in order to undestand the rest of the information. If not, you will be missing important facts. In addition, there are a lot of words used that are derived from this one, so you won't be able to undestand them.
Another important concept that gives a lot of information just by knowing it's meaning is "key distribution problems", they could be two or a thousand, it is important to know what they are and what they do.
The other question that seems important is the one of the "public key cryphtography" but this one can be inferred by the context.
The correct answer is A. This is the case of using comma before a coordinating conjunction <em>and</em> because it links two independent clauses. We can observe that there two parts of this full sentence and each part is independent from each other. Both of them have their own subject and verb. Therefore, we are using a comma before <em>and</em>.
The adjective is “approached”
