Suppose a particle is forced to move in the $x-y$ plane, under a constant magnetic field $\vec{B} = B\hat{z}$. The Hamiltonian can the be written as $$ H = \frac{\Pi^2}{2m} $$ where $\vec{\Pi} = \vec{P} - q\vec{A}$, where we choose the symmetric gauge $\vec{A} = \frac{B}{2}(-y,x,0)$. Using $\vec{\Pi}$, we can generate creation and annihilation operators $a = \frac{1}{\sqrt{2Bq\hbar}}\left(\Pi_x - i\Pi_y\right)$, such that $[a,a^{\dagger}] = 1$, and then $$ H = \hbar\omega_B\left(a^{\dagger}a + \frac{1}{2}\right) $$ Under this gauge, the operator $\widetilde{\Pi} = \vec{P} + q\vec{A}$ commutes with $\vec{\Pi}$, and we can also generate creation and annihilation operators $c = \frac{1}{\sqrt{2Bq\hbar}}\left(\widetilde{\Pi}_x - i\widetilde{\Pi}_y\right)$ such that $[c,c^{\dagger}] = 1$. Also, $c^{\dagger}c$ commutes with $[a^{\dagger},a]$, so we can find a common eigenbasis. This means that there are two quantum numbers, $n_a$ which are the eigenvalues of $a^{\dagger}a$ and $n_c$ which are the eigenvalues of $c^{\dagger}c$. Since the energy levels are dependent only on $n_a$, we conclude that there is infinite degeneracy in every level.
Now, the canonical angular momentum in the $z$ direction can be written as $L_z = -\hbar(a^{\dagger}a - c^{\dagger}c)$.
I am asked to explain the infinite degeneracy using symmetry.
The only symmetry I can see in this problem is rotations around the $z$ axis. I do notice that the energy levels are gauge-invariant, while the operators $c,c^{\dagger}$ are not, so there is no complete gauge symmetry. The fact that there is complete symmetry of rotations around $z$ cannot imply infinite degeneracy, as for example a $3D$ isotropic oscillator has complete symmetry of rotations around every axis, but does not have infinite degeneracy.
Which symmetry causes then the infinite degeneracy?