Explanation:

- The Eiffel Tower, like most structures, expands in response to temperature changes.

- Thermal expansion is when a material expands due to heat. Metals in the tower expand during summer, making it taller.

- Chemical expansion involves a change in material composition, not applicable to the tower's structure.

- Gravitational forces don't cause the tower to become taller; they affect the structural stability.

- Gradient isn't directly related to physical expansion due to temperature changes.

Answer: Option 1 - Thermal expansion is correct.