To multiply numbers in scientific notation, multiply the coefficients and add the exponents.
To multiply numbers in scientific notation, you multiply the coefficients and add the exponents. For example, to multiply (4.21 × 10⁻⁹) and (7.8 × 10⁻¹⁵), you multiply 4.21 by 7.8 to get 32.838 and add the exponents -9 and -15 to get -24. The final answer is 32.838 × 10⁻²⁴, in scientific notation.
So, the correct answer is d. 32.838 × 10⁻²⁴.