The valid options are #1 (not shooting) and #2 (shooting the ships of the active player).
Shooting space cannons is optional for any player (so #1 is always an option). Then, #2 is an option but not #3 or #4 because space cannons apply their hits to the ships of the active player, with the only exception being for the active player's space cannons.
Here are the relevant rules for Space Cannon Offense from the rules reference (my emphasis):
Beginning with the active player and proceeding clockwise, each player may use the “Space Cannon” ability of each of his units in the active system by rolling a specific number of dice for each of those units; these are called space cannon rolls.
If a player has the “Deep Space Cannon” technology, he can use the “Space Cannon” ability of his PDS units that are in systems that are adjacent to the active system. The hits are still assigned to units in the active system.
The active player must choose and destroy one of his ships in the active system for each hit result produced against his units.
If the active player is using the “Space Cannon” ability of his units, he chooses a player who has ships in the active system. That player must choose and destroy one of his ships in the active system for each hit the space cannon roll produced.
This is corroborated by the Learn to Play:
After the active player’s ships move into the active system, each player’s units in the active system may use their “Space Cannon” abilities against the active player’s ships in that system. The active player may also use his units’ “Space Cannon” abilities at this time.