Organisms that can interbreed and bear fertile offspring are considered to be in the same
a. class.
b. species.
c. order.
d. family.

B. species.

The organismic hierarchy of organism comes from the highest and lowest order as the following; 
1.    D Domain 
2.    K Kingdom 
3.    P Phylum 
4.    C Class 
5.    O Order 
6.    F Family 
7.    G Genus 
8.    S Species 

This hierarchy and order helped many taxonomist to determine and classify organisms by a variety of criteria. And it seems that species can breed the same fertile offspring.