Hey student!

Thanks for your wonderful questions! Let's see if I can explain better.

First, if you ignore the transition, then the sentence is a run-on sentence, right?

IC 1 = The resulting ridges in the land deflected the sound away from the ground.

Subject = ridges

main verb = deflected

Complete thought = The resulting ridges in the land deflected the sound away from the ground.

IC 2 = the researchers concluded that the most effective way to solve the airport's problem would be to make that feature of the landscape permanent.

subject = researchers

main verb = concluded

Complete thought = the researchers concluded that the most effective way to solve the airport's problem would be to make that feature of the landscape permanent.

So, we know that a transition is necessary, which means we can eliminate (D).

Next, the transitions "although" and "because" are called subordinating conjunctions. These conjunctions 1. create a dependent clause, which means that  2. they need an IC to be paired with it.

To answer your question, only subordinating conjunctions create dependent clauses. Coordinating conjunctions (For, And, Nor, But, Or, Yet, So) are used with a comma to connect two independent clauses. So these conjunctions do not form DCs.

ex. I ran to the store, and I bought some candy.

IC 1 = I ran to the store

IC 2 = I bought some candy

ex. I ran to the store, and although I bought some candy...

IC 1 = I ran to the store

DC = although I bought some candy

So this example is an incomplete sentence because IC 2 needs to appear after the DC.

The transition "however" is called a conjunctive adverb. These adverbs do not create a dependent clause. Instead, they are simply a nonessential part of an independent clause.

Ex. Jack is smart.  However, Jack is lazy. 

This is the same as:  Jack, however, is lazy.

Clearly, "However, Jack is lazy" is a complete sentence. The word "however" just creates a contrast with this sentence and the previous one.

So, when you plug in "however" into the sentence #20, a run-on sentence still exists.

That is why "however" can't be the correct answer.

Other conjunctive advers are "moreover," "furthermore," "therefore," "nevertheless," "thus," "unfortunately," etc...

Hopefully this clarifies things for you!!

Keep Moving Forward!! You got this!