I Believe it is C, Noun clause
A predicate adjective is the first adjective that follows a linking verb which connects two parts of a sentence (in this case, that verb is 'was'). So, the first adjective after was is 'sleek', so D is the correct answer.
Answer: supply and demand
Explanation:
I learned this earlier this year:
the amount of a commodity, product, or service available and the desire of buyers for it, considered as factors regulating its price.