Answer:
 Katherine paid $1.25 for 1 bottle of water.
Explanation:
What is the unit rate?
In this context, a unit rate is defined as the amount in dollars paid for one unit of a commodity.
In the given options, all the items are above one except 
option C( Katherine paid $1.25 for 1 bottle of water.)
 hence all the other amount does not correspond to the unit rate
 Mary paid $2.50 for 3 bottles of water. not a aunit rate
Ricardo paid $17.00 for 3 pumpkins.  not a unit rate
Katherine paid $1.25 for 1 bottle of water.  a unit rate
Joshua paid $6.00 for 3 masks.  not a unit rate