Cassandra is sitting cross-legged on her rug with her arms outstretched. She closes her eyes and makes a sound like "u." She continues this procedure. Cassandra is demonstrating and the sound is a(n) .

meditation, mantra

Explanation:

Meditation is a popular practice that is used to train the body and soul by redirecting and refocusing the mind. meditation can come in various ways like spiritual meditation, mantra meditation, etc. and posture which comes in many ways is essential to meditation. Mantra is an utterance, audible numinous sound utter during meditation associated to have spiritual power in which the audible numinous sounds create an effect, such as healing lower levels of stress and anxiety, transformation during spiritual or personal connection.

Therefore, from the above explanation, Cassandra is demonstrating meditation and the sound ("ummmm.") is a(n) mantra