Is trivial topology seperable? 
$(X,\mathcal T)$ is a t.s.
$\mathcal T                                                                                                                    = \{X,\emptyset\}$

How can we say space is seperable?

Yes trivial topology is seperable. We can say a topological space is seperable if and only if there exists at least one countable dense subset of $X$
If we consider singletons,
$\overline {\{x\}} = X$ and we know that singletons are countable thus trivial topology is seperable.
Besides any non-empty subset of $X$ is dense too
