Final answer:
Dirac proposed that if magnetic monopoles exist, all observable electric charges would have to be whole number multiples of the electron's charge. The term 'exact multiples' means that any charge would be an integer times the charge of an electron. While the concept of magnetic monopoles is compelling, they have not yet been observed.
Step-by-step explanation:
Dirac's insight on quantum mechanical magnetic monopole suggests that if even one such monopole existed, it would necessitate that electric charges in nature be exact multiples of a fundamental charge — the charge of an electron. To understand "exact multiples," consider the charge of an electron, which is approximately 1.60 × 10-19 C. If all electric charges are exact multiples of this charge, it means that every observable charge is an integer times this fundamental value. For instance, two electrons would have a charge of 2 × (1.60 × 10-19 C), and so on. The existence of a magnetic monopole is theorized but has not yet been observed experimentally, which challenges our current understanding of magnetism where all magnetic effects are attributed to electrical currents forming magnetic dipoles.