A great way to think about entropy is that entropy is how random and chaotic something is. The most random and chaotic phase of matter is gaseous, so whichever side of the balanced chemical equation has more moles of gas has more entropy.
Option A: C3H8(g) + 3O2(g) --> CO2(g) + 4H2O(l) has more moles of gas on the reactant side, so the reactants have more entropy than the products. A is wrong.
Option B: 2H2O(l) --> 2H2(g) + O2(g) has 3 moles of gas on the product side, and zero moles on the reactant side, meaning the products have more entropy than the reactants. B is correct. (But let's look at the others to confirm.)
Option C: 4Ag(s) + O2(g) --> 2Ag2O(s) has 1 mole of gas on the reactant side, and zero moles of gas on the product side, meaning the reactants have more entropy than the products. C is wrong.
Option D: S(s) + O2(g) --> SO2(g) has one mole of gas in both the product and reactants, so they have the same entropy. D is wrong.
In essence, the more moles of gas a reaction has on either side has more entropy, and the only reaction whose products contained more moles of gas than the reactants was equation B.