While 'line-of-sight' is a correct answer, not only is it subjective, it is also an incredibly vauge term to use.

The problem is that there is no easy way to explain it. Perhaps it is better to say 'line-of-sight
and it depends'.
First, it depends on how high the transmitting tower is and how much power it is putting out as to how far the signal can be received.
Next, it depends also depends on 'line-of-sight'. This is best described by way of an illustration:
If your transmitting tower is 150' AGL and you have some mountains that are 1500' AGL
higher than your tower, there is going to be an RF shadow behind those mountains. Effectively, blocking your 'line-of-sight' to that transmitting tower.
You can increase your ability to receive a faint signal by using an antenna that, by way of its design, is a high gain antenna. This is not the same as using a signal amplifier.