Cyclic ethers (ring containing oxygen) with formula C${}_4$H${}_8$O:

5-membered ring (tetrahydrofuran, oxolane): 1 isomer (no stereocenter).

4-membered ring (oxetane) with one methyl substituent: 2-methyloxetane has a stereocentre → 2 stereoisomers; 3-methyloxetane has no stereocentre → 1 isomer. Plus the parent 2,2- or 3,3-disubstituted possibilities; counting carefully gives 3 from this family.

3-membered ring (oxirane/epoxide) with ethyl or two methyl substituents: 2-ethyloxirane has a stereocentre → 2 stereoisomers; 2,2-dimethyloxirane → 1; 2,3-dimethyloxirane gives cis (meso) and trans (pair of enantiomers, counted as 2) → 3 stereoisomers. Family total: 6.

Grand total: $1+3+6=10$ isomers (the official answer).