Option 1: I, II and III are indeed essential for a self-sustained chain reaction.

I. More neutrons are released in each of the fission reactions - this is true, as each fission reaction releases more than one neutron, which in turn can hit other atoms, causing them to undergo fission.

II. The neutrons immediately take part in the fission process - this is also correct. The neutrons are not idle; they are very active and participate in other fission reactions, which is crucial for the chain reaction to be self-sustaining.

III. The fast neutrons are slowed down by Graphite - this is key in most reactors to make the neutrons more likely to hit other atoms and continue the chain reaction. Fast neutrons have a high energy and are less likely to cause fission, so slowing them down increases the efficiency.

Option 2: Leaves out II which is crucial for the reaction to continue.

Option 3: Leaves out I and III, both crucial for maintaining a chain reaction.

Option 4: This option falsely includes IV. Every neutron may not cause further fission, as neutrons can escape or be absorbed without causing fission.