Several classes of organisms would be classified into one..

a. order
b. genus
c. phylum
d. family

i think the answer is phylum because phylum is made up of classes and it says "several classes of organisms" but my book doesn't have answers in it.

You are correct .... c. phylum.


However, the question should mention CLOSELY RELATED some way. We would not put several DISTANTLY RELATED classes into one phylum. There's more to a phylum than just a bag to hold any old classes.

The classification system is (from largest to smallest):
domain, kingdom, phylum,class,order,family,genus,species

so classes would be grouped into phyla (singular for phylum)

Yep, its C.

Kingdom, Phylum, Class, Order, Family, Genus, Species (or Genus+Specific epithet)

The answer is C for phylum =)