formula is πV = nRT or π = MRT,   M is molarity

strength of solution is 1.25m

so 1000 gm water ..............................................1.25 mole sucrose

so 1000 gm water .............................................1.25*342=427.5gm sucrose

so 1427.5 gm solution ....................................1.25 mole sucrose

so 1427.5/1.34=1065.30 ml = 1.0653 L solution ...........................1.25 mole sucrose

so M = 1.25/1.0653

put all the values to get answer.