Answer:
(1 + 3i)(1 - 3i)
Step-by-step explanation:
The product of a complex number and its conjugate produce a real number.
given a ± bi the conjugate is a ∓ bi ← where a and b are real, then
(a + bi)(a - bi)
= a² - b² ← a real number
The only pair of conjugate complex numbers in the list is
(1 + 3i)(1 - 3i)
