Final answer:
In response to the command "at a close arm interval, dress right dress," the sailor should turn their head to the right while keeping their arms unchanged.
Step-by-step explanation:
Given the command "at a close arm interval, dress right dress", the sailor standing at the far right flank should respond by ensuring that their head turns to the right, while the arms remain unchanged. This command is used to align the formation by having each member turn their head to the right to ensure they are in line with the person beside them, maintaining a specified close interval. The arms remain along the side of the body and do not change position during this command.