keyuwang
Is "the kind of ... struggles" plural?? How come??
You need to check which part makes sense meaningwise
In California, a lack of genetic variation in the Argentine ant has allowed the species to spread widely;
due to their being so genetically similar to one another, the ants consider all their fellows to be a close relative and thus do not engage in the kind of fierce intercolony struggles that limits the spread of this species in its native Argentina.
(D) because they are so genetically similar to one another, the ants consider all their fellows to be close relatives and thus do not engage in the kind of fierce intercolony struggles that limit
To determine the difference, you can ask questions

different context would have different meanings). in this context:
1.)
Ask question what limit the spread of this species?
meaningwise: kind can not limit the spread .It doesn't make sense with kind.
2.) Engage in what?
Engage in kind
or engage in struggles/
( kind of just happens to be modifier)
engage in struggles make sense; not engage in kind.
Example:
I am engaged in physical exercise that can reduce weight--make sense
now I modify it, but core remain same
I am engaged in
type of physical exercise that can reduce weight--make sense
I am engaged in
typeS of physical exercise that can reduce weight--make sense
Other context:
You have to give me
colors that
are not same-
you have to give me colors
of a pen that are not same--> here meaning focus on colors not on pens.
I hope it helps.