(A) : The 19th electron in potassium atom enters into the 4s - orbital and not into the 3d- orbital.
(R) : (n + l) rule is followed for determining the orbitals of the lowest energy state.

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Both ‘A’ and ‘R’ are true and ‘R’ is the correct explanation of ‘A’.

‘A’ is true but ‘R’ is false

Both 'A' and 'R' are false

Both ‘A’ and ‘R’ are true and ‘R’ is not the correct explanation of ‘A’

answer is A.

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Detailed Solution

According to Aufbau's principle, the unfilled shell with lowest (n + l) value will be filled first. so 4s orbital is filled first than 3d orbital.
For 4s - orbital, (n + l) = 4 + 0 = 4
For 3d - orbital, (n + l) = 3+2 = 5

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