The correct answer is option 3: Rubidium.

In photoelectric cells, light energy is converted into electric energy. The filaments in these cells play a crucial role in this process. The filaments are made of a metal that has the ability to emit electrons when exposed to light.

Option 1: Tungsten is a commonly used metal in incandescent light bulbs due to its high melting point and durability. However, it is not typically used in the filaments of photoelectric cells.

Option 2: Copper is a good conductor of electricity but does not possess the property of emitting electrons when exposed to light. It is therefore not suitable for use in photoelectric cell filaments.

Option 4: Aluminium is also a good conductor of electricity but does not have the necessary characteristics for converting light energy into electric energy. It is not used in the filaments of photoelectric cells.

Option 3: Rubidium is the correct answer. Rubidium has the ability to emit electrons when exposed to light, making it an ideal metal for use in the filaments of photoelectric cells.