In Option A - INCORRECT
This statement begin with a past participle ''had'' which is use to talk about the past choice of the subject on intake of chocolates but was later compromised with the use of ''may'' when describing the present cost of the chocolate and now making it an optional object for the subject.

Furthermore Option B - INCORRECT
The beginning of this statement was correct with the indication of ''was once'' telling us of the past interest of the subjects as the noun phrase in this option, however, the use of ''but'' seeks to tell us of its expensive nature which might not still make the subjects loose interest.

However Option C - CORRECT.
This statement seeks to be the most correct, its sentence formation was simple and well explanatory with the beginning ''had once been''telling us of the interest of the subjects on the object and still a continuation of the interest was still maintained even as it tends to tell us of its scarcity and expensive rate with ''will make''afterwards.

Option D - INCORRECT.
The use of ''may'' at the later part of this option tends to speak on assumption on what i call truth, because everyone knows that the moment there is a shortage in the production of any goods, such goods seeks to be more expensive and scarce.

Option E - INCORRECT
This option feels uncertain about the shortage of cocoa and the use of ''may'' is use to prove this by saying it may not affect the expensive rate in the stock market.