In order to answer this question we will need first to set up the properly balanced equation, which the question already gave us:
C3H8 + 5 O2 -> 3 CO2 + 4 H2O
Now the molar ratio between C3H8 and H2O is 1:4, which means that for every 1 mol of C3H8, we will end up with 4 moles of H2O, now we have to find out the number of moles present in 15.0 grams of C3H8, we will do that by using its molar mass, 44.1 g/mol
44.1 g = 1 mol
15.0 g = x moles
x = 0.34 moles of C3H8
Now we have the number of moles of C3H8, and we can find the number of moles of H2O by using the molar ratio
1 C3H8 = 4 H2O
0.34 C3H8 = x H2O
x = 1.36 moles of H2O are produced