Which of the following is an unsupervised learning method?

a$.$ Naive Bayes

b$.$ KNN

c$.$ Random Forrest

d$.$ XGBoost

What is the goal of Principal Component Analysis $($PCA$)$ when applied to data?

a$.$ To categorize the data into distinct groups

b$.$ To reduce the dimensionality of the data while preserving as much variance as possible

c$.$ To maximize the covariance between variables

d$.$ To normalize the data distribution