the molarity of NaCl in the solution should be 1/3 i.e option B should be correct.0.25 M NaOH will act as a limiting reagent for the reaction between HCl and NaOH.
Assume volume of each before mixing is 1L
HCl + NaOH →NaCl + H2O
before reaction 0.50*1 0.25*1 0.75*1 --
after reaction 0.25 0 0.75+0.25
total volume after mixing = 1+1+1 =3L
Now NaCl =1 mol in 3L
so molarity is 1/3