As Kurosaki said, Concentration= moles/ volume. THerefore moles= concentration x volume. Remember in Most cases volume in in Litres in chemistry as seen by mol/L in concentration. Therefore to get number of moles of barium chloride: 0.05*0.5=0.025. Since 0.05 is volume in litres and 0.5 is concentration. The mole ratio shows that the ratio of molecules is number of mole ratio.
Therefore ratio is 1:1:1:2
However we are finding moles of barium sulfate and since ratio of barium sulfate:Barium Chloride is 1:1, the number of moles in barium chloride would be same as barium sulfate. therefore 0.025 . Ans =a
Yep hope that went into a little more detailed and helped you understand.