Explanation:

- The underlined word "vanguard" refers to the leaders or those at the forefront of any movement or trend, often paving the way for others.

- Option 1: Rearguard - This term refers to those positioned at the back, particularly in military terms, or the last group in a movement. It is the opposite of "vanguard."

- Option 2: Forerunner - Like "vanguard," it denotes someone or something that comes before others; a leader or pioneer.

- Option 3: Frontline - Often used to describe those at the most forward position in an activity, similar to "vanguard."

- Option 4: Backside - While it may imply being behind, it's not commonly used as a direct opposite to "vanguard."