How can we write the complex number 2 in polar form?
$r =\sqrt(2^2) = 2$
$z =2(\cos 0 + i \sin 0)$
The question is, do we use $\theta = 0$ or $\theta = 2 \pi$
All answers of the form $$2(\cos 2k\pi +i\sin 2k\pi)$$ are correct.
However, one typically has in mind a specific half-open interval of length $2\pi$ in which the polar angle $\theta$ should lie. For example, one may require $\theta\in(-\pi,\pi]$, or $\theta\in [0,2\pi)$, or $\theta\in (0,2\pi]$, etc., so the answer that is sought will depend on the interval required (which you have not specified).
They're equivalent of course, but your teacher probably wants the simplest form, theta = 0.