Step 1: Understanding the Question:
We need to identify which of the four listed tests is actually used to judge how effective a disinfectant is at killing microbes.
Step 2: Key Concept:
Disinfectant testing methods work by exposing a known number of test organisms to the disinfectant under controlled conditions and measuring how much of the organism is killed, often compared against a standard like phenol. This gives a phenol coefficient.
Step 3: Detailed Explanation:
The Widal test detects antibodies against Salmonella typhi in a patient's serum and is used only to diagnose typhoid fever.
The VRDL test detects antibodies against Treponema pallidum and is used only to screen for syphilis.
Neither test involves exposing microbes to a chemical agent, so neither can evaluate a disinfectant.
The Chick Martin test, in contrast, adds organic matter such as yeast suspension to the test system before applying the disinfectant, which mimics the presence of blood, pus, or feces in real use. This makes it a more realistic improvement over the older Rideal Walker test and a recognized method for rating disinfectant strength.
Because a valid test for this purpose does exist among the options, the choice of none of these is ruled out.
Step 4: Final Answer:
The Chick Martin test is the method used to evaluate disinfectants.