Final answer:
The instruction filling a branch delay slot must be independent of the branch, while the instruction for a load delay slot cannot use the data from the preceding load instruction.
Step-by-step explanation:
The main condition that must be satisfied by an instruction that is a candidate to fill a branch delay slot is that it must be independent of the branch itself. This means the instruction cannot depend on the result of the branch, nor can it affect the execution of the branch. Essentially, the instruction should be able to execute as if the branch were not there at all.
On the other hand, the main condition for an instruction to fill a load delay slot is that it cannot use the data being loaded by the load instruction that precedes it. This is because the data will not be available immediately, and any instruction that relies on this data would have to wait until the load completes, hence it cannot be used in the delay slot.