The correct answer is C. Each of the candidates choose a subject area: mechanical, electrical, or HVAC.
Explanation
A colon is a punctuation mark that has the function of pausing in the text to draw the reader's attention to the next text. It represents a longer pause than that of the comma and less than that of the period, so its pause is equivalent to the semicolon. Additionally the colon is used to precede an explanatory enumeration, to separate an exemplification from the rest of the sentence, among others. According to the above, the sentence "Each of the candidates choose a subject area: mechanical, electrical, or HVAC" is correct because in this sentence the most apropiate puntuation mark is the colon considering there is a list or explanatory enumeration "mechanical, electrical, or HVAC". So the answer is C.
