**Explanation:**

**Density and Buoyancy:**
The ability of a substance to float or sink in water depends on its density. Density is defined as the mass of a substance divided by its volume. If the density of a substance is less than the density of water, it will float. If the density of a substance is greater than the density of water, it will sink.

**Pair of Substances:**
Let's analyze each pair of substances given in the options:

a) Pin, oil drops:
- A pin is made of metal, which has a higher density than water. Therefore, a pin will sink in water.
- Oil drops, on the other hand, have a lower density than water and will float on its surface. However, oil drops are not a solid substance, so they cannot be compared directly to a pin.

b) Thermocol, Cotton thread:
- Thermocol is a lightweight material made of expanded polystyrene. It has a lower density than water, so it will float.
- Cotton thread is denser than water, so it will sink.

c) Plastic ball, feather:
- A plastic ball, depending on its size and material, can have a density either higher or lower than water. However, in this case, we can assume that the plastic ball has a lower density than water, which is why it floats.
- A feather is extremely lightweight and has a much lower density than water. Therefore, a feather will float.

d) Coin, rubber band:
- Coins are made of metal, which is denser than water. Therefore, a coin will sink.
- A rubber band is made of rubber, which has a lower density than water. Thus, a rubber band will float.

**Conclusion:**
Out of the given options, the pair of substances that will both float in water is a plastic ball and a feather (option C). Both the plastic ball and the feather have a lower density than water, allowing them to float on its surface.