The correct answer is c) supposed.
The verb "supposed" means "expected or obliged to do something." In this sentence, it is being used to indicate that car drivers are legally required to wear seat belts.
The other options are not as appropriate.
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Seemed implies that car drivers may not actually wear seat belts, but only appear to do so.
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Supposing suggests that car drivers are not necessarily required to wear seat belts, but rather that it is only an idea or possibility.
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Meaning means "having a particular intention or purpose." It is not the correct word to use in this context.
Therefore, the only option that accurately conveys the meaning of the sentence is supposed.