The correct answer is option 'D' - all (a), (b), and (c). In a continuous crystallizer, the size of the crystals formed is influenced by several factors including the rate of heat transfer, the degree of turbulence, and the degree of supersaturation. Let's discuss each of these factors in detail:

Rate of heat transfer:
- Heat transfer plays a crucial role in determining the crystal size in a continuous crystallizer.
- The rate of heat transfer affects the cooling or heating of the solution, which in turn affects the solubility of the solute.
- Faster heat transfer leads to faster cooling or heating, resulting in a higher degree of supersaturation and smaller crystal size.
- Slower heat transfer, on the other hand, leads to slower cooling or heating, resulting in a lower degree of supersaturation and larger crystal size.

Degree of turbulence:
- Turbulence refers to the chaotic and irregular motion of the fluid in a crystallizer.
- Higher turbulence promotes better mixing and enhances mass transfer within the system.
- Increased mixing helps in distributing the solute more uniformly, preventing the formation of large crystals.
- In the presence of turbulence, the solute molecules have a higher chance of encountering the seed crystals, leading to the growth of smaller crystals.
- Therefore, a higher degree of turbulence promotes the formation of smaller crystal sizes.

Degree of supersaturation:
- Supersaturation is a measure of how much the solute concentration exceeds its equilibrium solubility.
- Higher supersaturation levels result in faster nucleation and growth of crystals.
- When the degree of supersaturation is high, the solute molecules are more likely to come out of the solution and form crystals.
- This leads to the formation of smaller crystal sizes.
- Conversely, lower supersaturation levels result in slower crystal growth and the formation of larger crystals.

In conclusion, the size of crystals in a continuous crystallizer depends on the rate of heat transfer, the degree of turbulence, and the degree of supersaturation. These factors collectively influence the nucleation and growth of crystals, determining their final size.