Final answer:
In C++, to write 2.1 raised to the power of 2.4, you use the function call pow(2.1, 2.4), where 'pow' is the power function from the math library.
Step-by-step explanation:
To write 2.1 to the power of 2.4 in C++, the correct function call is pow(2.1, 2.4). This uses the pow function from the math library, where the first argument is the base and the second argument is the exponent. So the correct answer is B. pow(2.1, 2.4).