Answer:
2.f
Step-by-step explanation:
At a particular energy level, say energy level 4. So lets compare subshells within the particular shell or energy level. Comparing 4s, 4p, 4d and 4f. Subshell s has the lowest energy and subshell f has the highest energy. So 4s orbital will be filled first, followed by 4p, 4d and 4f. This is in accrdance to Aufbau principle.
Therefore the f subshell is likely to be filled after the rest of the sub shells at a particular energy level are filled.