Final answer:
The f antigen occurs when the c and e alleles of the RhCE gene are in the cis position on the same chromosome.
Step-by-step explanation:
The "f antigen" of the Rh blood group system is a complex antigenic site on the red blood cell surface which arises when the RhCE gene's c and e alleles are in cis position to each other, meaning they are on the same chromosome.
Therefore, the f antigen occurs under the condition when c and e are cis (option 3). This is different from other antigen patterns in the Rh system or other blood group systems like the ABO system.