The equation given is \( y = 0.35x \), where \( y \) represents the total cost and \( x \) represents the number of pieces of candy.
To find the cost of one piece of candy, we substitute \( x = 1 \) into the equation:
\[ y = 0.35(1) = 0.35 \]
So, the cost of one piece of candy is \( 0.35 \) or \( $0.35 \).
The correct answer is:
C. $0.35