Final answer:
The third step's missing reason in your geometric proof could be that vertical angles are congruent, which is a common principle in such proofs. However, additional context is required to confirm this.
Step-by-step explanation:
The student has provided some given angles within a geometric figure and asked to prove that x = 24°. With the given information, we infer that there is a relationship between angles ∠EDF, ∠ADB, and ∠BDC, likely within a polygon such as a quadrilateral. Unfortunately, the reference information provided is not relevant to this geometric situation, as it talks about the width of the Moon and other unrelated data. For the student's question, the missing reason in step 3 is not specified, but Vertical angles are congruent (option a) is a common reason used in proofs involving intersecting lines. The angles sum rule in a triangle, which states that the sum of the angles in any triangle is always 180°, might also be relevant in a complete geometric proof that includes these angles.