The correct answer is: C. 120 meters
A block overlap is a technique used in multiple access communications to ensure that all users have a chance to transmit data. In a single line MACLS, each user is assigned a unique block of frequencies. However, because of the finite speed of light, there is a delay between when a user transmits a signal and when that signal is received by the other users. This delay can cause collisions, where two users transmit at the same time and their signals interfere with each other.
To prevent collisions, a block overlap is used. This means that each user’s block of frequencies is slightly longer than the actual data that they are transmitting. This extra time allows for the delay between transmission and reception, so that the other users can receive the signal without interference.
The amount of block overlap required depends on the speed of light and the length of the communication link. For a single line MACLS, the required block overlap is 120 meters.
Option A is incorrect because it is the length of a single block of frequencies. Option B is incorrect because it is the length of a single transmission. Option D is incorrect because it is no block overlap, which would result in collisions.