Explanation:

- C. Friendship is one of the most coveted relationships on earth.

- This sentence introduces the topic of friendship.

- B. Like all relationships, it is also governed by some well-defined principles.

- This builds on the topic by explaining that friendship follows principles.

- A. The first of these states that the only way to have a friend is to be one.

- This specifies one of those principles.

- D. Thus, one must be reliable as a friend to expect reliability in others.

- This concludes by emphasizing the principle of mutual reliability.

Option: 2 - CBAD presents the correct sequence as it logically flows from introducing friendship to describing its principles.