The correct answer is option 2: transpiration. Transpiration is the process by which plants lose water from their leaves through tiny openings called stomata. When water is lost through transpiration, it creates a suction force that pulls water up from the roots through the plant stem and into the leaves.

Option 1: osmosis is the movement of water across a semipermeable membrane from an area of higher water concentration to an area of lower water concentration. While osmosis does play a role in water movement within plants, it is not the ultimate cause of water movement against gravity.

Option 3: photosynthesis is the process by which plants convert sunlight, water, and carbon dioxide into glucose and oxygen. While photosynthesis requires water, it is not directly responsible for water movement against gravity.

Option 4: diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration. While diffusion does occur in plants, it is not the main driving force behind water movement against gravity.

In conclusion, the ultimate cause of water movement in a plant stem against gravity is transpiration.