Three of the options have something in common: the scientists are using an instrument to measure or manipulate something:
Stopwatches measure the amount of time it takes for a swimmer to go one lap in the pool.
A telescope is used to see the moons orbiting Jupiter.
A saw is used to cut a board in half.
here the stopwatch, telescope and a saw are uses as instruments.
in the remaining options, the marbles represent something, and a model is exactly that: a representation of an idea, so the correct answer is Marbles inside a hollow sphere - they are a model of a nucleus.