Which of the following best supports the concept that matter is made-up of particles?

The correct option is A drop of ink spreads uniformly in water.

To understand why this supports the concept that matter is made up of particles, we can break down the observation using the kinetic molecular theory of matter:

1. Existence of Particles and Intermolecular Spaces:
Both the ink and the water are composed of extremely tiny, individual particles. Because matter is not a continuous, solid block, there are empty spaces (intermolecular spaces) between the water molecules and between the ink particles.

2. Constant Random Motion (Diffusion):
The particles of matter are in continuous, random motion. When a drop of ink is placed in water, the moving ink particles collide with the moving water molecules. Due to these collisions, the ink particles gradually move from an area of higher concentration (where the drop was initially placed) to areas of lower concentration throughout the water. This process of intermixing of particles of two different types of matter on their own is called diffusion.

3. Uniform Distribution:
Over time, this continuous random motion ensures that the ink particles distribute themselves evenly throughout the spaces between the water molecules, resulting in the ink spreading uniformly. This uniform spreading would be impossible if matter were continuous and not made up of discrete, moving particles.

Let us briefly look at why the other options do not directly support this specific particulate nature of matter:
- Water boiling at 100°C and Ice melting to form water represent phase transitions (physical changes of state) associated with temperature and thermal energy, which do not uniquely demonstrate the discrete particulate nature as clearly as diffusion does.
- Metals conducting electricity is a property related to the movement of free electrons, which is a specific electrical property rather than a general demonstration of the particulate nature of bulk matter.