Final answer:
C) Software design documentation is the key tool for software designers, developers, and their testing teams. Hence, option C) is correct.
Step-by-step explanation:
The key tool for software designers, developers, and their testing teams is Software design documentation (option C).
Software design documentation includes detailed descriptions of the overall design, the architectural patterns used, data structures, algorithms, and any other relevant technical information.
This documentation serves as a guide for developers to understand and implement the software design accurately.