Conduction:
Conduction is a mode of heat transfer that occurs through direct molecular collision between objects or particles in contact with each other. In this process, the heat energy is transferred from the higher temperature region to the lower temperature region within a solid, liquid, or gas.

Molecular Collision:
Molecular collision refers to the interaction between individual molecules, where they collide with each other and transfer energy. These collisions occur due to the random thermal motion of molecules.

Heat Transfer by Molecular Collision:
When two objects or particles are in contact with each other, the higher temperature object transfers heat energy to the lower temperature object through molecular collision. The molecules of the hotter object have higher kinetic energy and collide with the molecules of the cooler object. During these collisions, the molecules transfer their kinetic energy to the cooler molecules, increasing their temperature.

Conduction is the primary mechanism of heat transfer in solids, where the molecules are closely packed, and molecular collisions occur more frequently. Metals, for example, are excellent conductors of heat due to the high mobility of electrons, which facilitates the transfer of thermal energy.

Comparison with other modes of heat transfer:
- Convection: Convection is the transfer of heat energy through the movement of fluid (liquid or gas) particles. While convection can also involve molecular collisions, it is primarily driven by bulk fluid movement. Therefore, molecular collision is not the main mechanism of heat transfer in convection.
- Radiation: Radiation is the transfer of heat energy through electromagnetic waves. It does not require a medium for propagation and can occur in vacuum. Unlike conduction, radiation does not involve direct molecular collision for heat transfer.
- Scattering: Scattering refers to the redirection of light or other electromagnetic radiation by particles or molecules. It is not a mode of heat transfer and does not involve molecular collision for transferring thermal energy.

Conclusion:
The correct answer is option 'B' - conduction. Heat transfer by molecular collision occurs through conduction, where heat energy is transferred from a higher temperature region to a lower temperature region through direct molecular collisions between objects or particles in contact with each other.