Final answer:
In the Drake Equation, 'fnow' is the fraction of planets with civilizations that currently exist, allowing for the possibility of detecting these civilizations through current observational efforts. It is important for considering timing and synchronization for detectability.
Step-by-step explanation:
In the context of the Drake Equation, fnow represents the fraction of planets with civilizations on which the civilizations exist now. Specifically, it is option A) the fraction of planets with civilizations on which the civilizations exist now. The Drake Equation attempts to estimate the number of active, communicative extraterrestrial civilizations in the Milky Way galaxy. To do this, the equation multiplies several factors related to the development of life, intelligence, and civilization. Knowledge of exoplanets and assumptions about life's prevalence across the cosmos are used to make educated estimates using the equation.
The term fnow is crucial because it considers the timing aspect - civilizations that might have existed at other times would not be detectable now if they have since ceased to be active or if they will only develop in the future. Hence, fnow focuses on the synchronization of these civilizations' existence with our current time, enabling the possibility of their detection through efforts like SETI (Search for Extraterrestrial Intelligence).