Solution
First seven natural numbers are : 1, 2, 3, 4, 5, 6, 7
As we know,
$$average\ =\ \frac{Sum\ of\ observations}{number\ of\ observations}$$
$$\ =\ \frac{1^2+2^2+3^2+4^2+5^2+6^2+7^2}{7}$$
$$\ =\ \frac{1+4+9+16+25+36+49}{7}$$
$$\ =\ \frac{140}{7}=20$$
Hence, Option D is correct.
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