The Maxwell's four equations are written as:

(i) $\oint \vec{E} . \vec{ds}=\frac{q_0}{\varepsilon_0}$
(ii) $\oint \vec{B} . \vec{ds}=0$
(iii) $\oint \vec{E} . \vec{dl}=\frac{d}{dt} \oint \vec{B} . \vec{ds}$
(iv) $\oint \vec{B} . \vec{d s}=\mu_0 \varepsilon_0 \frac{d}{d t} \oint \vec{E} . \vec{d s}$

Out of four Maxwell's equations above, which one shows non-existence of monopoles?

(ii) only

Had there been a monopole, the integral of B.ds on a closed surface enclosing the monopole would have been non-zero.