Concept:
Analyzing words relating to productivity and systemic operational performance allows us to pair them with their corresponding negative states.
Step 1: Define the target word.
* EFFICACIOUS: Successful in producing a desired or intended result; effective. For example: "The vaccine proved to be highly efficacious."
Step 2: Map out the choices.
* fruitful: Producing good or abundant results; highly productive (Synonym).
* feasible: Possible to do easily or conveniently (Related to viability).
* practical: Concerned with the actual doing or use of something rather than with theory (Related to utility).
* unproductive: Not producing positive, useful, or desired results.
Step 3: Conclude the absolute antonym.
* Since efficacious denotes delivering successful, positive results, the term unproductive represents its direct functional opposite.