Which one of the following is the correct sequence in increasing complexity ?

The correct sequence representing increasing biological complexity is Protein → Tissue → Organ → Organism. This hierarchy follows the levels of biological organization:

• Protein: These are large biomolecules (macromolecules) consisting of amino acids. They represent the molecular level of organization.
• Tissue: A group of similar cells and their intercellular substances that work together to perform a specific function (e.g., muscular tissue).
• Organ: Different types of tissues integrate to form an organ, which performs more complex physiological functions (e.g., the heart or stomach).
• Organism: This is the highest level of complexity in the given options, representing an individual living being (like a human) formed by the coordination of various organ systems.

Thus, Option D correctly arranges these components from the simplest molecular building blocks to the most complex integrated individual.