What type of sentence is listed below?

"The system worked, but the books were heavy and slow to read."
A
simple
B
compound
C
complex
D
compound-complex

I suggest that it's simple becoz compound sentences are like this

EX:because the soup was too cold I warmed it in the microwave

and complex

EX:though I prefer drinking coffee I drank tea today.

so it's completely different from ur sentences so ur sentence is a simple one is what I suggest u

in june, we will have been married for eighteen months. (i am assuming this is what you want? the sentence, as you have presented it, is grammatically incorrect--   you want it corrected, yes? )