Hope this helps you with your answer.
NaCl --> Na+ + Cl-
c= n/V
n= c*V= 1.0 mol dm^-3 * 0,250 dm^3= 0,25 mol (mol dm^-3= M; mL= cm^3)
but you need to multiply that with 2 be cause you have 0,25 moles of Na+ and 0,25 moles of Cl- so the answer is 3) 0,50 moles
;)
ps you need to take into account both species be cause from 1 mole of NaCl you get 1 mole of Cl- and one mole of Na+ and when you add that up you get 2 moles
