Final answer:
The exact value of Hubble's constant isn't known due to the complex and dynamic cosmic expansion over time, which affects the constant and our ability to measure long-distance scales accurately.
Step-by-step explanation:
One reason Hubble's constant isn't exactly known today is c) The complex nature of galactic motion. The Hubble constant is instrumental in estimating the expansion rate of the Universe by relating the recession speed of galaxies to their distances. However, assuming that the Hubble constant has remained unchanged since the Big Bang is problematic, as observations have indicated that the expansion rate of the Universe may not have been constant over time. Astronomers originally thought the expansion was slowing down due to gravity but discovered that the expansion rate might actually be accelerating, which introduces significant uncertainty in the value of the Hubble constant. Recent estimates of this constant range from about 19.9 km/s per million light-years to 23 km/s per million light-years, showing a considerable improvement from half a century ago, where the uncertainty was a factor of 2. This demonstrates the progress made in astrophysics and cosmology, but also the complexity of understanding the universe's expansion. Still, the exact value of the Hubble constant remains elusive due to the dynamic nature of cosmic expansion, influenced by factors such as dark energy and gravity.