When two complex conjugates are multiplied together, the result is always a non-negative real number. This is because if you have a complex number \( a + bi \), its conjugate is \( a - bi \). When you multiply them:
\[ (a + bi)(a - bi) = a^2 - (bi)^2 = a^2 + b^2 \]
where \( a^2 + b^2 \) is a non-negative real number.
Now, let's examine the given options:
- 16i - This is an imaginary number (not a real number).
- 25 + 4i - This is a complex number (not purely real).
- 9 − 9i - This is also a complex number (not purely real).
- 20 - This is a positive real number.
Therefore, the only answer that could be the result of two complex conjugates being multiplied together is:
20.