Final answer:
Facial recognition is considered the most invasive authentication technique for user privacy among the given options.
Step-by-step explanation:
The most invasive authentication technique for user privacy among the given options is facial recognition. Facial recognition involves analyzing and identifying a person's unique facial features to verify their identity.
However, this technique raises concerns about privacy as it requires capturing and storing an individual's biometric data, which some argue can easily be misused or compromised.