What is the correct way to write one million as a power of $10?$

The correct option is A ${10}^6$
$\text{One Million}$

$=1,000,000$

$=10\times 10\times 10\times 10\times 10\times 10$
$(\because 6\text{zeros in one million)}$

$={10}^6$
$(\because 10\text{multiplied by itself}6\text{times)}$

Hence, one million can be represented as a power of $10$ as $\underset{–}{{10}^6}$, i.e., option (a.) is the correct one.