Brackish water is commonly found in:

The correct answer is Estuaries. Brackish water is water that has more salinity than freshwater but less than seawater. It is most commonly found in estuaries, where rivers meet the sea and fresh water mixes with salt water. This creates a unique environment with varying salinity levels. Among the options:

• Lakes: Most lakes are freshwater, though some are saline (like the Dead Sea).
• Glaciers: Contain fresh water (ice).
• Estuaries: Correct; this is where freshwater and saltwater mix, creating brackish conditions.
• Rivers: Typically contain fresh water, not brackish.

Hence, the correct answer is "Estuaries".