An aqueous solution of a weak acid and its conjugate base in known as a

This solution is called a buffer.

Explanation

It is an aqueous solution containing a weak acid and its conjugate base or a weak base and its conjugate acid. A buffer's pH changes very little when a small amount of strong acid or base is added to it. It is used to prevent any change in the pH of a solution, regardless of solute..

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The correct answer is a buffer

Explanation:

A buffer is an aqueous substance that resists change in pH when little quantity of a strong acid or strong base is added. A buffer is made up of a weak acid and its conjugate base or vice versa. Examples of buffer include Ammonium buffer, phosphate buffer and carbonate buffer.

NOTE:

A conjugate acid is a substance formed after receiving proton (from an acid) by a base.

A conjugate base is the substance formed/left when an acid loses a hydrogen ion after it dissociates.