Ants, by themselves, are weak creatures, but
when ants are considered along with others of their species, they are known for their highly organized and powerful colonies and nests, which sometimes consist of millions of individuals.
A. when ants are considered along with others of their species,
--> correct
B. when
considering them with others of their species, then
-->
when +
then --> redundant.
considering seems to modify
ants -->
ants considering ants --> illogical.
C. ants,
when considered with others of their species,
--> remove non-essential modifier
when considered with others of their species, we have
ants they are known ..., we have the subject
they that stands next to its antecedent
ants --> redudant and illogical.
D. considering them with others of their species,
-->
considering seems to modify
ants -->
ants considering ants --> illogical.
E. considered with others of their species, then
-->
comma + conjuntion (
, but) requires Subject + Verb --> lack of subject.