Astudent mixed together aqueous solutions of y and z. a white precipitate(solid)formed. which could not be y and z y z (a)hydrochloric acid silver nitrate(b)hydrochloric acid sodium hydroxide (c)calcium chloride. silver nitrate(d)sodium chloride. silver nitrate

(A)hydrochloric acid + silver nitrate
HCl(aq) + AgNO3(aq) > AgCl(s) +HNO3(aq)

(B)hydrochloric acid + sodium hydroxide 
HCl(aq) + NaOH(aq) > NaCl(aq) + H2O(l)

(C)calcium chloride + silver nitrate
CaCl2(aq) + AgNO3(aq) > AgCl(s) +Ca(NO3)2(aq)

(D)sodium chloride + silver nitrate
NaCl(aq) +  AgNO3(aq) > AgCl(s) +NaNO32(aq)

AgCl is a white precipitate.
In (B) no precipitate was formed, so answer is B.