Indentured Servants I believe
The best reason that the demand for manual labor increased in Colonial America was because the Explorers discovered new territories.
<h3>What new territories were discovered?</h3>
The British officers discovered the new areas that were good for agricultural practices for growing tobacco, cotton etc, especially in the Southern colonies.
Hence, this best reason that the demand for manual labor increased in Colonial America was because the Explorers discovered new territories for agricultural practices.
Therefore, the Option D is correct.
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<em>I believe it is the Factory System</em>