Final answer:
A machine designed to count the number of grains in a sample is known as a Seed counter. The total plate count method is used for measuring bacterial contamination in food samples such as peanut butter. So the correct answer is Option A.
Step-by-step explanation:
The machine used to determine the number of grains in a sample is called a Seed counter. This device is specifically designed to accurately count the number of seeds or grains in a given sample. It is commonly used in agricultural research and seed distribution to ensure precise measurements and efficient processing. A similar task in microbiology, specifically measuring the concentration of bacterial contamination in processed peanut butter, can be achieved by using a method like total plate count, which allows for viable bacteria to grow on a nutrient plate so they can be counted. Turbidity measurement and direct counting under a microscope are other methods, but they serve different purposes and contexts.