The correct answer is C, 1=excystation; 2=division by binary fission; and 4=encystation.  Cyst, the infective stage, excyst in the host's intestine and two trophozoites per cyst emerge upon excystation (event 1).  The trophozoites multiply by binary fission or mitosis (event 2) and attach to the intestinal mocuosa by means of the ventral sucking disc.  At a certain point the trophozoites encyst to originate the infective cyst (event 4).  The cyst is shed in the stool and can contaminate water and food, which are the main vehicles for the transmission of giardiasis.