Concept:The first person to define speed was Galileo Galilei, who expressed speed as the distance covered per unit of time.
Explanation:Galileo introduced the quantitative definition of speed: the magnitude of change in position per unit of time.
He is credited with being the first to measure speed in this way, making speed a scalar quantity.
Among the other options:
- Newton is known for laws of motion and universal gravitation, not the first definition of speed.
- Kepler formulated the three laws of planetary motion about elliptical orbits.
- Ptolemy was an ancient astronomer who described the geocentric model.
Thus, only Galileo matches the description of the first person to define speed.
Answer:Galileo (Option D)