Mindful of the importance of varying sentence length in a paragraph or essay, spiro sometimes chooses to use to demonstrate relationships between equally important ideas.
a. compound-complex
b. complex
c. simple-compound
d. compound

I'd go with A, Compound-complex.
A Compound-complex sentence is a sentence with two or more coordinate independent clauses and one or more dependent clauses.
This would be fitting to use to demonstarte relsationships between equally important ideas, because if they are both equally important, each should have it's own independent clause
(i actually don't know anything i'm just guessing by going with the definitions?)
(just don't quote me on this)

A, to visit her father at his army camp