The question asks for the missing step in the proof, where AB is proven parallel to DF.

From the given problem:

• We know that point B is the midpoint of segment EF, and point A is the midpoint of segment ED.
• Step 2 calculates the midpoints of DE and EF using the midpoint formula.
• Step 3 calculates the slopes of line segments AB and DF.
• Step 4 states that the slopes of AB and DF are equal using the Substitution Property of Equality.

The final step (Step 5) concludes that AB is parallel to DF.

To justify this conclusion, we need to define parallel lines mathematically. The definition of parallel lines states that two lines are parallel if and only if their slopes are equal.

Thus, the missing step in the proof is:

B. Definition of parallel lines

Would you like me to explain any part of this in further detail? Here are 5 related questions:

1. What is the definition of the slope of a line?
2. How do you calculate the midpoint of a segment?
3. What does the Substitution Property of Equality mean in geometry?
4. What is the relationship between equal slopes and parallel lines?
5. How can you prove two segments are congruent using midpoints?

Tip: When solving geometry proofs, carefully match each statement with the appropriate definition, property, or theorem!