Answer:
The correct answer is option  A) The poem that I read in class is by Maya Angelou.
Step-by-step explanation:
 Let's start by clarifying what it is a subordinate clause. 
A subordinate clause is that part of a sentence that if we wrote alone would not make sense. It is something we use to link an idea into a sentence. 
All subordinate clauses must have a subject and a verb. 
To make it clearer let's look at the example: 
The poem that I read in class is by Maya Angelou. 
In this case, "that I read" is a relative pronoun (which is part of the subordinate clauses) and which serves to link the subject with an idea: that I read in class. 
If we only wrote a sentence that says "that I read in class is by Maya Angelou" the sentence would not make sense and something would be missing. 
Therefore, the correct answer is option A.