The best prediction for the second part of Mark's experiment is:
The wagon will have more kinetic energy than when it was empty.
This is because kinetic energy (KE) is calculated using the formula \( KE = \frac{1}{2} mv^2 \), where \( m \) is mass and \( v \) is velocity. When Mark adds a bag of sand, the mass of the wagon increases, leading to an increase in kinetic energy, as long as the velocity remains the same or increases.