a) The key decision in designing data models for MongoDB applications revolves around the structure of documents and how the application represents relationships between data
b) There are two tools that allow applications to represent these relationships: references and embedded documents
c) When designing data models, always consider the application usage of the data (i.e. queries, updates, and processing of the data) as well as the inherent structure of the data itself
d) All of the mentioned

Answer: d
Explanation: Denormalized data models allow applications to retrieve and manipulate related data in a single database operation.