Understanding Vocabulary in Context: 'Sanguine'

Read the following passage:

In the quiet of the library, students huddled over their books, poring over every detail, readying themselves for the upcoming examination. Their brows furrowed in concentration, they were determined to imbibe as much knowledge as possible. Outside, the world was bustling with life, indifferent to the frantic efforts inside. One student, however, appeared unfazed; her demeanor was sanguine, as if the impending test held no weight over her serenity.

As the clock ticked down, an air of tension permeated the room. The sounds of turning pages and the occasional whisper seemed amplified in the stillness. Yet, amidst this cacophony of anxious energy, she remained tranquil, encapsulated in her own universe of understanding.

What does the word 'sanguine' most likely mean as used in the passage?