a level chemistry

Some 1.0 mol dm–3 solutions were mixed using equal volumes of each solution.
Which pair of solutions would give the greatest mass of solid?
A Ba(OH)2 and MgCl2
B Ba(OH)2 and MgSO4
C Ba(OH)2 and NaCl
D Ba(OH)2 and Na2SO4

I am also struggling with this questions right now. I'm thinking the answer would be the compounds with the greatest mr because concentration and volume are the same so the thing that must be different is their moles so as mass=mrxmoles the greatest mr would be the answer but that would give an answer of C.

You have the molar volume mol/dm³ and you need to convert it to molar mass g/mol and if you got it, you can finally calculate the mass with the molar mass by the help of the known moles of the compounds:

molar mass * moles = mass.