1. (b) That's right. The general rule is that an agent must perform the tasks undertaken and do so in accordance with the principal's instructions.
Whether or not an agent has a discretion to act depends on how specific the instructions are. If the agent has a general authority to represent the principal, a discretion will exist as to whether or not to carry out a particular act. The agent must exercise that discretion as if he were acting for himself. B seems to have been given a general authority in relation to the overall maintenance of the building.
However, in relation to the painting of the outside of the building, the instructions are much more specific - to paint the building every three years. In the case of specific instructions, the agent does not have any discretion and must do whatever the principal has said must be done. If the agent fails to carry out the principal's instructions, the agent will be liable to the principal for losses caused.