Select the more acidic member of each of the following pairs: SnO and $SnO_2$

In general, with an increase in electronegativity of the central atom, the oxide becomes more acidic and with a decrease of electronegativity, the oxide becomes more basic.

Electronegativity increases going up the group and from left to right in the period, which is why the oxides of metals are mostly basic (d-block metals can have acidic as well as basic oxides because of the wide variety of oxidation numbers), nonmetal oxides are acidic and the ones in the middle of the two, such as aluminium, are amphoteric.

Regarding the oxidation state, since the acidity of a cation rises rapidly with charge (Lewis acid strenght), the higher the oxidation number, the more acidic the oxide will be.

In $\mathrm{SnO}$, Sn is +2.

In $\mathrm{SnO_2}$, Sn is +4.

Therefore, $\mathrm{SnO_2}$ is more acidic.