Step 1: List the haemopoietic organs in the order they take over.
Blood cell formation shifts location as the fetus grows. It starts in the yolk sac, then moves to the liver, then the spleen joins in for a while, and finally the bone marrow takes over for good.
Step 2: Fix the spleen's window on this timeline.
The spleen works as a blood forming organ only for a limited stretch of fetal life, from the early months up to about the fifth month of gestation.
Step 3: Explain why the handover happens at this point.
Once the skeleton has formed enough marrow cavity, the bone marrow is ready to take over blood formation for good. This handover is placed around the fifth month, so the spleen's haemopoietic job ends there.
Step 4: Check the other options against this timeline.
The 3rd month is too early, the spleen is still active then. The 7th and 9th months are too late, bone marrow has already taken over by then.
Final Answer:
The spleen works as a haemopoietic organ until the fifth month of gestation, option (2).