Final answer:
The acid test refers to a crucial and definitive test of success or viability.
Step-by-step explanation:
The phrase 'the acid test' refers to option c) A crucial and definitive test of success or viability. In chemistry, an acid is a compound that produces hydrogen ions (H+) when dissolved in water. The term 'acid test' is often used metaphorically to describe a test or examination that determines the true nature or quality of something.