Answer:
A. Mo
Step-by-step explanation:
The given electron configuration: 1s22s22p63s23p64s23d104p65s24d4 shows 42 electrons and Molybdenum has atomic number 42.
But the correct electronic configuration of molybdenum is [Kr]4d5 5s1, and not [Kr]5s24d4 which is based on the diagonal rule.
The diagonal rule states that a half full 5s subshell and half-full 4d subshell are more stable than a full 5s subshell and 4d filled with four electrons.
Hence, the correct element is "A. Mo".