Explanation:

A volatile variable refers to a variable whose value can change unexpectedly due to external factors such as hardware changes, multi-threading, or signal handlers. Such variables are typically mutable, meaning they can be modified during program execution.

Other options:

• (A) Volatile variable → While "volatile" is a keyword in some languages like C and Java, it does not define a variable type; rather, it is a qualifier.
• (C) Immutable variable → Incorrect, as immutable variables cannot change once assigned.
• (D) Dynamic variable → This usually refers to variables that are allocated dynamically in memory but does not specifically relate to volatility.

Thus, the correct answer is (B) Mutable variable.