What is specifically referred to as a controlled block?

Answer: B — series of blocks between signals

A controlled block refers to a series of blocks between signals that are managed by signaling systems to ensure safe train operations. These blocks allow for the regulation of train movements, helping to prevent collisions and maintain service schedules. Within a controlled block, signals denote when it is safe for a train to proceed and which track sections are clear. In contrast, the other options do not accurately describe what a controlled block is. For instance, using a block exclusively for passenger trains does not encompass the broader functionality and safety management associated with controlled blocks. Similarly, categorizing a series of blocks between signals as a freight transportation area neglects the distinction between freight and passenger operations, which is irrelevant to the definition of a controlled block. Lastly, areas without signal control signify situations where movements are not managed by signals, directly contradicting the concept of controlled blocks, which are defined by their use of signaling to control train movement.