Which term signifies being fully aware of something?

The term that signifies being fully aware of something is "cognizant." When someone is described as cognizant, it means they have knowledge and awareness of a particular situation, fact, or subject. This word conveys an understanding and recognition of important details or circumstances, highlighting the person's active engagement and informed state regarding the topic at hand.

In contrast, the other terms convey a lack of awareness or engagement. "Unaware" suggests not having knowledge or being oblivious to something. "Indifferent" typically implies a lack of interest or concern, while "inattentive" refers to not paying attention or being careless in regard to the matter. Therefore, "cognizant" is the term that most accurately captures the essence of being fully aware.