The correct answer is D because autoencoders are an unsupervised machine learning method commonly used for anomaly detection when labeled data is not available.

From AWS documentation:

" Autoencoders learn to compress and reconstruct input data. During anomaly detection, they learn normal patterns in data. Data points that the model cannot accurately reconstruct are flagged as anomalies. "

This approach is ideal when there is no labeled data and when patterns must be learned based on normal behavior alone — a common situation in IoT sensor data environments.

Explanation of other options:

A. Linear regression requires labeled data and is used for predicting continuous values.

B. Classification requires labeled data to assign inputs into categories.

C. Decision trees are supervised learning models and also require labeled datasets.

Referenced AWS AI/ML Documents and Study Guides:

AWS Machine Learning Specialty Guide – Unsupervised Learning Techniques

Amazon SageMaker Examples – Anomaly Detection Using Autoencoders