This question is asking you to find which answer choice is "most strongly supported" by the passage. Essentially, this means the passage provides the premises, and you're looking for the answer choice which provides A POSSIBLE conclusion. It's important to remember that "most strongly supported" does not mean that the conclusion will be required.
First, let's break down the passage into its component parts:
Premise: Modern brushless car washes use cloth strip "mitters" rather than brushes. Premise: "Mitters" are easier on the car's finishes then the brushes. Premise: This is important for clear-coat finishes on cars today which are more easily scratched than older finishes. Conclusion: ???
Then, turn to the answer choices: (A) is incorrect. The premises do not tell us that older cars with tough finishes were scratched by the brushes.
(B) is incorrect. The premises do not tell us anything about why the car washes were introduced.
(C) is correct. While this seems to be a slight divergence from where the premises were headed, it is supported by the premises. The premises tell us that "mitters" are better on car finishes then brushes, and that older finishes are tougher than new ones. Therefore, we can infer that "mitters" are less likely to scratch older finishes than the new clear-coat finishes. Premise 3 has told us that the use of mitters is important for not scratching clear coat finishes.
(D) is incorrect. The premises do not talk about effectiveness or a preference for brushes.
(E) is incorrect. This adds in an assumption about "more cars in use today." We do not know how many of each type of car are in use.