Final answer:
In 'A Wrinkle in Time', the Black Thing first appears as a giant shadow when viewed by Meg, Charles Wallace, and Calvin.
Step-by-step explanation:
The Black Thing takes the form of A) a giant shadow when Meg, Charles Wallace, and Calvin first see it. This entity is described in 'A Wrinkle in Time' as a dark and menacing presence that spreads across the sky, covering stars and planets, an immense cloud of darkness that threatens to engulf all hope and light.
It is the embodiment of evil in the novel, menacing the protagonists and symbolizing the darkness that they must overcome through their journey.