Explanation:

- Option 1: Sperm

- Sperm cells are among the smallest cells in the human body.

- Option 2: Ovum

- The ovum, or egg cell, is the largest cell in humans. It's visible to the naked eye, about 0.1 mm in diameter.

- Option 3: Neurons

- Neurons can be very long. For example, some stretch from the spine to the toes, but their cell body is much smaller than the ovum.

- Option 4: Osteoclast

- Osteoclasts are large, multinucleated cells that break down bone tissue, but they are smaller than the ovum in overall size.

The largest cell in the human body is the ovum.