The growth of small regional kingdoms in the history of southern India was influenced by several factors. One significant factor was the absence of large empires, which allowed smaller political entities to emerge and flourish. However, among the given options, the most relevant factor for the growth of small regional kingdoms in southern India was the scarcity of manpower.

Southern India, particularly during ancient and medieval times, faced challenges related to a relatively lower population density compared to other regions. This scarcity of manpower meant that it was difficult to sustain large empires with expansive territories. Instead, smaller kingdoms emerged, which were more manageable in terms of administration and defense.

The smaller regional kingdoms often focused on consolidating their local power bases, exploiting available resources, and maintaining control over smaller territories. They were able to effectively govern their limited areas, establish trade networks, and engage in regional politics.

While the other factors mentioned in the options (absence of minerals like iron, too many divisions in the social structure, and absence of vast areas of fertile land) may have had some influence on the political landscape, the scarcity of manpower was a key factor in shaping the history of southern India and the growth of small regional kingdoms.