Explanation:

- The word "vanguard" refers to the people at the forefront or leading position in a movement, activity, or field, especially in innovation or progress.

- Let's look at the options:

- Option 1: Rearguard – This means the group of people positioned at the back, often providing protection, but in this context, it's the opposite of being at the front or leading.

- Option 2: Frontrunner – This is a person who is leading or likely to win; similar in meaning to "vanguard."

- Option 3: Frontline – This refers to those who are in the leading position; also similar to "vanguard."

- Option 4: Backside – This refers to the physical back part of something, and is not a precise antonym for "vanguard."

- The correct answer is Option 1: Rearguard.