Final answer:
The correct password for the secret passageway on the Grand Staircase that led to the Clock Tower Courtyard in the Harry Potter series is 'Mimbulus Mimbletonia'. It refers to a magical plant in the series.
Step-by-step explanation:
The password for the secret passageway on the Grand Staircase that led to the Clock Tower Courtyard in the Harry Potter series was c) Mimbulus Mimbletonia. This passphrase was cleverly chosen as it refers to a plant native to the Harry Potter world, known for its defensive mechanism that resembles a Stinkhorn fungus and can squirt Stinksap as a defense. This secret passageway was one among many concealed within the walls of Hogwarts School of Witchcraft and Wizardry, each requiring a unique password to gain access.