"To most, the word architecture connotes a grandeur typically associated with the Old World— flying buttresses, Doric columns, baroque flourishes, byzantine arabesques— and thus many of the more _____ structures, especially those not obviously inspired by neoclassicism, are often thought to be cobbled together haphazardly instead of following some prescribed architectural idiom."

Meaning: So basically usually the word architecture showcases grandeur and so many of the (opposite of grandeur) structures are combined together haphazardly. The word we are looking for should be on the lines of not grand.

A- Modest means humble or unassuming. Correct choice and works well with our intended meaning.
B- Secular means non-religious. Illogical in context.
C- Unassuming means restrained/modest. Good pair word here. Correct choice.
D- Dilapidated means run down or ruin, neither of which make sense within the context.
E- Ramshackle means state of despair or ruin. Again illogical like D.
F- Sedate means calm. Totally out of scope.