The ground state shorthand notation for manganese (Mn) is [Ar] 4s2 3d5.
Manganese has an atomic number of 25, which means it has 25 electrons. To write its electron configuration using shorthand notation, we start with the nearest noble gas preceding manganese, which is argon (Ar). Thus, we begin with [Ar].
After argon, manganese continues filling up the electrons in the following orbitals:
- The 4s orbital fills first, receiving 2 electrons: 4s2.
- Following that, the 3d orbital is filled with the remaining 5 electrons: 3d5.
Combining these, we get the complete shorthand notation: [Ar] 4s2 3d5. Therefore, the correct option is a) Kr 4s2 3d5.