Explanation:

The fifth generation of computers, which began in the 1980s and continues to evolve, is primarily characterized by its focus on Artificial Intelligence (AI). This generation aims to develop computers that can understand natural language, learn, make decisions, and perform tasks that typically require human intelligence. While computers of this generation can be (a) 'Used in home' and some might eventually achieve (c) 'Very low cost', these are not the *defining* characteristics. The core objective and technological advancement driving the fifth generation is the integration and development of AI capabilities. Therefore, Artificial Intelligence ($ \text{b} $) is the key characteristic.