**Explanation:**

The correct answer is option 'A' (Walk) because it is different from the other options in terms of the mode of transportation. Let's analyze each option in detail to understand why.

a) **Walk**: Walking is a mode of transportation where a person moves by using their legs. It does not involve any external means of transport, such as vehicles or animals. Walking is a natural and basic way of getting from one place to another, commonly used for short distances.

b) **Sail**: Sailing refers to the act of traveling in a boat or ship using the power of wind. It involves the use of sails to catch the wind and propel the vessel forward. Sailboats are specifically designed to harness the wind's energy and navigate through water bodies like seas, rivers, and lakes.

c) **Ride**: Riding generally refers to traveling on an animal, bicycle, or motorcycle. It involves sitting or standing on a vehicle or animal and using it to move from one place to another. Riding can also include using public transportation like buses, trains, or trams.

d) **Drive**: Driving is the act of operating a vehicle, such as a car, truck, or motorcycle, to transport oneself or others. It involves sitting behind the wheel, controlling the vehicle's movements, and using the engine's power to move forward or backward.

In comparison to the other options, walking stands out as it does not involve any external means of transport. While sailing, riding, and driving all require the use of some form of vehicle or animal, walking relies solely on a person's own physical ability to move.

Therefore, option 'A' (Walk) is the odd one out as it does not involve the use of an external mode of transportation.