C) There is most likely a planet orbiting that star
First, think about what would cause a star to appear dimmer for a short period of time. The most likely conclusion is that something is blocking part of the light from the star. That something may be relatively close to the star, or extremely distant. If it's distant, it's unlikely to happen more than once per occluding object and hence be unpredictable. If it happens regularly and predictably, it's extremely likely that the occluding object is in orbit around the star and that the plane of the orbit is such that the object regularly occludes the star from Earth's point of view. So let's take a look at the options and see what makes sense.
A) There is probably not a planet orbiting that star
* This is definitely not correct since the above description almost certainly describes a planet. Or at least strongly implies that there is a planet orbiting the star. So this is a bad choice.
B) There is another star orbiting that star
* Unlikely unless the other star is far dimmer than the star being observed. There's a more likely situation available.
C) There is most likely a planet orbiting that star
* This is highly probable, So this is the correct choice.
D) none of the above
* Since we've picked "C", that means that this is a bad choice.
