A convex lens is a lens that possesses at least one surface that curves outwards. It is shaped like a dome with one or both surfaces curved. Convex lenses can be made of transparent materials that can refract (bend) light.

Option 1 is Aluminium. This is a metal and is not transparent, and so it cannot be used to make a convex lens as it does not refract light.

Option 2 is Glass. This is a very common material to make lenses from because it is transparent and has good optical properties.

Option 3 is Sapphire. This is a type of gem which is also transparent and can therefore be used to make a convex lens.

Option 4 is Water. In a sense, water can be shaped into a lens (like a raindrop), and because it is transparent, it can be used to make a simple temporary type of lens.

Therefore, the correct answer is Aluminium (Option 1) because it is not transparent and cannot refract light.