multiply (-8 radical 21) (-7 radical 2). give an exact answer please

To multiply \((-8 \sqrt{21})\) and \((-7 \sqrt{2})\), we can follow these steps:

1. Multiply the coefficients: \(-8 \times -7 = 56\).
2. Multiply the radicals: \(\sqrt{21} \times \sqrt{2} = \sqrt{42}\).

Putting it all together, we have:

\[ (-8 \sqrt{21}) \times (-7 \sqrt{2}) = 56 \sqrt{42} \]

Therefore, the exact answer is:

D. \(56 \sqrt{42}\)