The non-essential clause is underlined in sentence C. "Its coloring, which is brown and green, allows it to hide in marshy areas". This clause is non-essential because it can be dropped without chaning the meaning of the sentence. The information it contains is not considered essential for the sentence to be complete.
Its coloring allows it to hide in marshy areas.
As you can see, the non-essential clause can be removed and the sentence still expresses a complete thought and the same meaning as the original sentence.
What is more, the essential clause is underlined in sentence D. "The person who sells souvenirs is in the lobby". In this case, the relative clause is essential because the information it contains is necessary to identify the person referred to in the sentence. This is not any person who is in the lobby but the one who sells souvenirs. If you remove the clause, the meaning of the sentence is changed.
The person is in the lobby.
The information contained by the relative clause is essential. And, as you can see, the clause is not between commas.