Introduction:
Planes are a popular means of transportation due to their speed and efficiency. In this question, we are asked to choose the most suitable adjective to describe planes as a means of traveling.

Explanation:
To determine the correct adjective, we need to consider the comparative forms of adjectives. Adjectives can be in three forms: positive, comparative, and superlative.

- Positive form: This is the base form of the adjective, without any comparison. For example, "convenient" is the positive form of the adjective.

- Comparative form: This is used when we want to compare two things or actions. It is formed by adding "-er" to the positive form or by adding "more" before the positive form. For example, "more convenient" is the comparative form of the adjective "convenient."

- Superlative form: This is used when we want to compare more than two things or actions and indicate the highest degree or quality. It is formed by adding "-est" to the positive form or by adding "most" before the positive form. For example, "most convenient" is the superlative form of the adjective "convenient."

Answer:
In this question, we are asked to choose the most suitable adjective to describe planes as the means of traveling.

The correct answer is option 'c' - "most convenient."

Planes are considered the most convenient means of traveling because they are the fastest and most efficient mode of transportation. They allow people to travel long distances in a shorter time compared to other means of transportation such as trains, cars, or buses.

Conclusion:
In conclusion, planes are the most convenient means of traveling among all the options given. This is because they offer the fastest and most efficient mode of transportation.