**Core Concept**
Pasteurization is a process used to kill harmful microorganisms in liquids, particularly in milk and other dairy products. The efficiency of pasteurization can be assessed by measuring the reduction in viable pathogens, which indicates the effectiveness of the process.

**Why the Correct Answer is Right**
The correct method to assess the efficiency of pasteurization is by using the **Bacillus stearothermophilus** spore test. This is because B. stearothermophilus is a thermophilic bacterium that forms highly resistant spores, which are difficult to kill. The presence or absence of these spores after pasteurization indicates whether the process was successful. The spores are used as a surrogate marker to evaluate the efficiency of pasteurization, as they are more resistant to heat than other pathogens.

**Why Each Wrong Option is Incorrect**
**Option A:** The D-value is a measure of the time required to kill a certain percentage of microorganisms at a given temperature, but it does not directly assess the efficiency of pasteurization.
**Option B:** The Z-value is a measure of the change in temperature required to alter the D-value by a factor of 10, but it is not a direct measure of pasteurization efficiency.
**Option C:** The plate count method involves counting the number of viable bacteria in a sample, but it is not a specific or reliable method for assessing pasteurization efficiency.

**Clinical Pearl / High-Yield Fact**
When evaluating the efficiency of pasteurization, it is essential to use a surrogate marker like B. stearothermophilus spores to ensure that the process has been effective in killing pathogens and preventing foodborne illness.

**Correct Answer:** C.