The Pony express was the mail service that used horses as the mail service. The word supplanted in the excerpt means replaced. Thus, option c is correct.
Supplanted is a word that means to replace or displace a thing, person, or object with another. The prior work or object is superseded by another when they are better than the other.
In the Pony express, the mail delivery service conducted by the horses was replaced by the transcontinental telegraph as they were a better and more efficient way of communication than the horses' service.
Supplanted is the word used in the sentence when something wants to be replaced by another thing that is better, and more useful than the other. Supplanted cannot mean defeated, joined, or unearthed.
Therefore, option c. replaced is the correct meaning of the word supplanted.
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