Step 1: Understand the expression.
The phrase member of a band of robbers refers to a person who is part of an organized group of robbers, especially one that attacks people in groups. We need the exact vocabulary word that expresses this idea.
Step 2: Analyze the options.
A dacoit is a robber, especially one belonging to an armed gang in India. It is close in meaning, but the standard English one-word substitution commonly used for a member of a band of robbers is brigand.
A brigand means a member of a gang or band of robbers living by plunder, especially in forests or mountains. This matches the meaning most precisely.
A thief is simply a person who steals. This word is too general and does not indicate membership in a band.
A pirate is a robber who attacks ships at sea. This meaning is completely different.
Step 3: Conclusion.
Hence, the correct one-word substitution for member of a band of robbers is brigand.