The electronic configuration of actinoids cannot be assigned with degree of certainty because of

small energy difference between 5f and 6d levels

The correct answer is option 3. small energy difference between 5f and 6d levels.

The electronic configuration of actinoids cannot be assigned with certainty because of the small energy difference between 5f and 6d orbitals.
In actinoids, the 5f and 6d orbitals have nearly the same energy. Because of this, electrons can occupy either of these orbitals, leading to irregular and uncertain configurations.