Log boat:
- A log boat is a primitive type of boat that is made by hollowing out a log.
- It is propelled by paddling or rowing using oars or paddles.
- The speed of a log boat depends on the strength and speed of the person rowing it.
- It is generally a slow-moving boat due to its heavy and bulky nature.

Motor boat:
- A motor boat is a type of boat that is powered by an engine.
- It uses a propeller to move through the water and can achieve higher speeds compared to other types of boats.
- The speed of a motor boat depends on the power of the engine and the design of the boat.
- Motor boats can reach speeds of up to 60 miles per hour or more, depending on their size and horsepower.

Oar boat:
- An oar boat is a type of boat that is propelled by rowing using oars.
- Oars are long poles with blades at one end that are used to push against the water and propel the boat forward.
- The speed of an oar boat depends on the strength and endurance of the rower.
- It is generally slower than a motor boat but can still achieve decent speeds depending on the rower's ability.

All of these:
- The statement "All of these" suggests that all three types of boats mentioned (log boat, motor boat, and oar boat) have a speed of 4 km/hour.
- However, this statement is not accurate as the speed of these boats can vary significantly depending on various factors such as design, power source, and the strength of the person operating the boat.
- While it is possible for a log boat, motor boat, or oar boat to have a speed of 4 km/hour, it is not a fixed speed for all boats of these types.

Correct answer:
- The correct answer is option 'A' - log boat.
- Log boats, due to their heavy and bulky nature, are generally slower compared to motor boats and oar boats.
- They are not designed for speed but rather for stability and transportation over short distances.
- It is important to note that the speed of any type of boat can vary depending on various factors, and it is not accurate to generalize a fixed speed for all boats of a particular type.