Yes, because $3$ and $4$ share no common factors besides $1$.

No, because they can both occur if the result is $12$.

No, because the set of outcomes for $A$ is $\{3, 6, 9\}$ and for $B$ is $\{4, 8\}$, which have no common elements.

Yes, because the sum of probabilities is $\frac{3}{10} + \frac{2}{10} = 0.5 < 1$.