What is unsupervised learning?Number of groups may be knownFeatures of group explicitly statedNone of the aboveNeither feature nor number of groups is known

Unsupervised learning is a type of machine learning that looks for previously undetected patterns in a data set with no pre-existing labels and with a minimum of human supervision. In contrast to supervised learning that usually makes use of human-labeled data, unsupervised learning, also known as self-organization allows for modeling of probability densities over inputs.

The correct answer to your question is "Neither feature nor number of groups is known". This is because in unsupervised learning, the algorithm is not provided with any prior information about the data, i.e., the features of the group are not explicitly stated, and the number of groups or clusters is also not known. The algorithm itself determines the structure and patterns in the data.