Final answer:
A tennis ball is the closest in size to a stellar black hole, which is incredibly dense and features an event horizon wrapping around a singularity much smaller than a tennis ball.
Step-by-step explanation:
The object closest in size to a black hole formed from the collapse of a star is a tennis ball. Black holes that are formed from the gravitational collapse of stars have incredibly dense cores, known as singularities, which are wrapped by an area called the event horizon. For the Earth to become a black hole, it would have to be compressed to a radius of only 1 centimeter, which is less than the size of a tennis ball. A typical stellar black hole is much smaller in size compared to a mountain or the Earth.