Explanation:

Post MySQL 5.5 (including MySQL 6.0 and beyond), the utf8 character set was replaced by a new character set called utf8mb4. The original utf8 character set in MySQL was capable of storing only up to 3 bytes per character, which was insufficient to store certain characters in the Unicode standard (e.g., some emoji and other characters outside the Basic Multilingual Plane).

The new utf8mb4 character set was introduced to support the full Unicode character set, using up to 4 bytes for each character, which allows it to store all possible Unicode characters, including those outside the Basic Multilingual Plane (BMP).

• utf8 (prior to MySQL 6.0) used 3 bytes for each character.
• utf8mb4 (introduced as a more complete UTF-8 encoding) uses 4 bytes for each character.

Thus, the correct answer to the question is (D) 4 bytes.