Final answer:
All three, Jim, Bob, and Ron, can be charged with conspiracy since they discussed committing a robbery and took a step towards execution of the crime by purchasing ski masks.
Step-by-step explanation:
Jim, Bob, and Ron can all be charged with conspiracy to commit robbery if it is demonstrated that they had an agreement to engage in the criminal act and took an overt act in furtherance of the conspiracy, such as purchasing ski masks that Ron had done. Conspiracy is a charge that does not require the substantive crime (the robbery itself) to be completed; the agreement and any step towards committing the crime is sufficient for the charge. In the scenario presented, they all appear to be involved in the planning of the robbery, and at least one affirmative step toward committing the crime has been taken, so there seems to be enough ground to charge all three with conspiracy.