Final answer:
The word 'serene' is both associated with peacefulness and with the mythical bird the phoenix, fitting the description provided in the question.a) Serene
Step-by-step explanation:
The word serene is the one that means "peaceful" and can be found before "days" to imply a state of tranquility. Moreover, serene is also associated with the legendary bird the phoenix, symbolizing renewal and calmness.
None of the other options provided, such as Harpy, Quiescent, or Pegasus, correctly fit into the context of the question as they either do not mean 'peaceful' or they do not precede 'days' to form a common expression.