What statement best filling in the first blank of this proof and completes the sentence: "it is given "

C.

Step-by-step explanation:  

We have been given that angle A is a right angle and angle B is a right angle. We are asked to choose the correct option that completes the first statement of the proof.

Let us prove that step by step.

First blank:

We have been given that .

Second blank:  

This means that and

Third blank:

By the definition of right angles a right angle is an angle of exactly 90°.

Last blank:  

Substitution property of equality states that if

Hence, by the substitution property of equality .

Therefore, the statement completes the first blank of the proof and option C is the correct choice.