The basic structural and functional unit of both plant and animal tissues is:

The correct answer is Cell. The cell is the basic structural and functional unit of life in all living organisms, both plants and animals. Tissues are composed of groups of cells that work together to perform specific functions. Among the options:

• Nitrogen: A chemical element, not a structural unit of tissues.
• Cell: Correct; it is the fundamental unit of life.
• Oxygen: A chemical element, not a structural unit.
• Hydrogen: A chemical element, not a structural unit.

Hence, the correct answer is "Cell".