The correct answer is option 1: curvature of the earth.

Television signals are transmitted in the form of electromagnetic waves, which travel in straight lines. However, the earth is curved, and this curvature affects the reception of television signals. As the signal travels from the broadcast antenna, it follows the curvature of the earth. Beyond a certain distance, known as the line of sight, the curvature prevents the signal from reaching the receiver.

Option 2, weakness of antenna, is not the correct answer. While a weak antenna can result in a weaker signal, it does not determine the distance at which the television signal cannot be received.

Option 3, weakness of signal, is also not accurate. The strength of the signal can decrease as distance increases, but it is not the main reason why television signals cannot be received beyond a specific distance.

Option 4, absorption of the signal in the air, is not the correct answer either. While the atmosphere can cause some signal degradation, it is not the primary reason for the limited distance of television signal reception.

In conclusion, the curvature of the earth is the primary factor that limits the distance at which television signals can be received.