Which one of the following does not contain electrons in 5f subshells
A.$Th(Z = 90)$
B.$Lr(Z = 103)$
C.$U(Z = 92)$
D.$Cm(Z = 96)$

Hint: To answer this question, you should recall the concept of laws which govern the filling of atomic orbitals. The laws which dictate the filling of electrons in atomic orbitals are Aufbau principle, Hund's Rule of Maximum Multiplicity and Pauli’s exclusion principle.

Complete step by step answer:
The electronic configuration of elements is based on majorly 3 rules:
According to Pauli's exclusion principle in an atom, no two electrons will have an identical set or the same quantum numbers. There salient rules of Pauli Exclusion Principle are that only two electrons can occupy the same orbital and the two electrons that are present in the same orbital should be having opposite spins.
According to Hund’s Rule of Maximum Multiplicity rule for a given electronic configuration of an atom, the electron with maximum multiplicity falls lowest in energy.
According to the Aufbau principle, the electrons will start occupying the orbitals with lower energies before occupying higher energy orbitals.
The configuration of the elements can be written as:
\[Th\]:\[{\left[ {Rn} \right]^{86}}6{d^{27}}{s^2}\],
$Lr:\left[ {Rn} \right]{\text{ }}5{f^{14}}7{s^2}\;7{p^1}$,
\[U:\left[ {Rn} \right]{\text{ }}5{f^3}{\text{ }}6{d^1}{\text{ }}7{s^2}\]
$Cm:\left[ {Rn} \right]5{f^7}$.
The configuration is \[{\left[ {Rn} \right]^{86}}6{d^{27}}{s^2}\]according to Aufbau principle, 5f subshell cannot be filled before \[6d\] and \[7s\]. So \[Th\] does not contain a 5f subshell.

Hence, the correct option is option A.

Note:
 Exceptions of Aufbau Rule. The electron configuration of chromium is \[\left[ {Ar} \right]3{d^5}4{s^1}\] and not \[\left[ {Ar} \right]3{d^4}4{s^2}\](as suggested by the Aufbau principle). This exception is attributed to several factors such as the increased stability provided by half-filled subshells and the relatively low energy gap between the 3d and the 4s subshells.