d. We can easily move our hand in air, but to do the same through a solid block of wood, we need a karate expert because the particles of air have large intermolecular spaces and least force of attraction between them. Thus, one can easily move the hand in air and push the particles of air apart. However, the particles of solid have minimum amount of intermolecular space and maximum force of attraction between them. Thus, a greater amount of force is required to move the particles of solid apart. Thus, a much greater force is required to move hand through a solid block of wood.