If the string costs c cents per foot, then it costs 3c cents per yard (because 1 yard = 3 feet). So the price of y yards of the string will be $3cy$ cents. Dividing this by 100 (to convert from cents per dollars), the cost of the string will be $\frac{3cy}{100}$ dollars, choice (D).

Note: a real ETS question should mention the conversion between yard and feet.

Regards