Choice C is correct. The total cost to purchase the shirt is the $$20$ cost of the shirt plus the $5$% sales tax. The value of the $5$% sales tax on the $520$ shirt is equivalent to(0.05)(20)$$, or S1$. Therefore, the total cost to purchase the shirt is $$20 +$1, or \$21$.

Choice A is incorrect and may result from neglecting to multiply by $920$ when finding the value of the sales tax.

Choice B is incorrect and may result from dividing by $10$ , instead of by $100$, and then neglecting to multiply by $$20$ when finding the sales tax. Choice D is incorrect and may result from interpreting the sales tax of $5$% as $55$.