Although the listening audience of satellite radio amount to a number

IMO C too, less than ONE percent is correct, lower than one percent , naaa

Thank you all for responding. OA is "C"

so how do we distinguish between using?
less vs. lower vs. fewer

+1 for (C)
The "...listening audience" is countable,so " less than" is the most appropriate.Also,"... owners mounted a law suit last year" is PAST tense, so "amounted to " is the right fit.

less is used for non countable nouns, can someone give a better reasoning

This is a debatable topic - less/greater/fewer. My 2 cents.

Lower is not used a lot -
The listening audience amounted to lower than one percent of the population. - incorrect
The level of water at 5 am was lower than that at 4:00 AM. - correct

Less is used in groups/subsets, fractions, money, periods of time and distance, distance and numerical data.
The listening audience amounted to less than one percent of the population. - fraction comparison
Danny always carries less than two dollars in his wallet. - money comparison
Dallas has a population of fewer than five million and less immigrants than ever before.

if you are talking about an increase in a single statistic, you use GREATER. Use ‘greater than’ when describing numbers ALONE.
The number of apples is greater than the number of oranges
The amount of water is greater than the amount of juice
The clarity of water is greater than that of juice

Use fewer when you can count
The listening audience amounted to fewer than one percent of the population. - incorrect
The count of listening audience was fewer than the count of the population. - correct

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