At which point does a substance with a fixed volume and shape physically change into a substance with a fixed volume and undefined shape?

A. boiling point
B. melting point
C. freezing point
D. vaporation point

Ok so co(g) is not pronoused that. the only clue is that youpronouse it the oppisote way. 

Hi + naoh -> h2o + nal

The question is incomplete. complete question is attached belowanswer: correct answer is option b  i.e.  solid particles  reason: filter paper has small pores, through which smaller particles can pass through. however, solid particles  have relatively larger size. hence, they are least likely to pass through.