Which of the following is the most complex according to their cellular level of organization?

Analyze the question: The question asks which option has the highest level of organization. The levels, from simplest to most complex, are: cell → tissue → organ → organ system → organism.

Explain the concept:

• Euglena is a unicellular organism. It has only the cellular level of organization, though it carries out all life processes within that one cell.
• Hydra is a multicellular organism (a cnidarian) with a tissue level of organization. It has an epidermis and a gastrodermis, with a simple nerve net, but no true organs.
• Hair is a structure made of dead, keratinized cells. It is a part of the skin, an epidermal outgrowth, and not a true organ.
• Heart is an organ made up of several tissues (cardiac muscle, connective tissue, epithelial tissue, nerve tissue). It is part of the circulatory system.

Solution (ranking by complexity):

1. Euglena: cellular (unicellular)
2. Hydra: tissue level (an entire organism, but a simple one)
3. Hair: a simple structure of keratinized cells
4. Heart: organ level, made of several tissues working together

Taken strictly as levels of organization, the heart is the highest-level structure listed: it is an organ composed of multiple tissues. Hydra is a whole organism, but its body plan is only at the tissue level of organization.

Answer: A. Heart

Tip: For "level of organization" questions, remember the order: Cell → Tissue → Organ → Organ system → Organism. Ask yourself what level each option reaches. Be careful not to choose a whole organism like Hydra just because it is "alive and complete". The question is about the structural level, not whether it is a full organism.