Maximum oxidation state is shown by Os (option A).

Explanation:
The maximum oxidation state refers to the highest possible positive charge that an atom can acquire when it loses or shares its valence electrons. The elements in the periodic table exhibit a range of oxidation states, depending on their electronic configuration and the number of valence electrons.

Comparison of the elements:
To determine which element has the maximum oxidation state among Os (Osmium), Co (Cobalt), Cr (Chromium), and Mn (Manganese), we can compare their electronic configurations and examine their highest possible oxidation states.

1. Os (Osmium):
Osmium is a transition metal with the atomic number 76. Its electronic configuration is [Xe] 4f14 5d6 6s2. Osmium can exhibit oxidation states ranging from -2 to +8. The highest oxidation state of Os is +8, which is achieved when it loses all its valence electrons from the 5d orbital.

2. Co (Cobalt):
Cobalt is also a transition metal with the atomic number 27. Its electronic configuration is [Ar] 3d7 4s2. Cobalt can have oxidation states ranging from -4 to +3. The highest oxidation state of Co is +3, which is achieved when it loses all its valence electrons from the 3d orbital.

3. Cr (Chromium):
Chromium is another transition metal with the atomic number 24. Its electronic configuration is [Ar] 3d5 4s1. Chromium can exhibit oxidation states ranging from -4 to +6. The highest oxidation state of Cr is +6, which is achieved when it loses all its valence electrons from the 3d orbital.

4. Mn (Manganese):
Manganese is a transition metal with the atomic number 25. Its electronic configuration is [Ar] 3d5 4s2. Manganese can have oxidation states ranging from -3 to +7. The highest oxidation state of Mn is +7, which is achieved when it loses all its valence electrons from the 3d orbital.

Conclusion:
Comparing the highest possible oxidation states of Os, Co, Cr, and Mn, we find that Osmium (Os) has the maximum oxidation state of +8. Therefore, the correct answer is option A, Os.