Final answer:
The recommended following distance under 50km/h is the 3-second rule, which should be extended based on driving conditions and visibility. The options provided do not directly answer the question as posed.
Step-by-step explanation:
The recommended following distance under 50km/h is not specifically listed among the options provided. Generally, the rule of thumb for a safe following distance is the 3-second rule, which can be adjusted based on driving conditions, speed, and the driver's experience. The 3-second rule suggests that when driving under good conditions, you should stay at least three seconds behind the vehicle in front of you. This would give you enough time to react and stop if necessary. To check whether you are following this rule, you can pick a fixed point that the vehicle in front passes and count the seconds until you reach the same point. If you count less than three seconds, you're following too closely.
However, in lower visibility, adverse weather conditions, or when following larger vehicles, this following distance should be increased. So, for speeds of 50km/h, a 3-second following distance is typically recommended under good driving conditions, although it may be larger than options given in the question.