Answer
Correct answer is define and measure the speed
Explanation
Galileo was the first scientist who define speed and give the formula of speed as distance over time
discover radioactivity
This option is also not correct because Radioactivity was discovered by Henri Becquerel in 1852.
invent the reflecting telescope
this option is not correct because reflecting telescope was discovered by Sir Isaac Newton in 1668
propose the heliocentric theory:
This option is also not correct because Nicolaus Copernicus proposed the
heliocentric theory in 1543
