Step 1: Understanding the Concept:
Ripening index measures the extent of cheese ripening.
It is based on protein breakdown.
Step 2: Key Formula or Approach:
Ripening Index = (% Soluble Protein % Total Protein) $\times$ 100.
Step 3: Detailed Explanation:
During cheese ripening, proteins are broken down.
This increases the soluble protein fraction.
The ripening index is the ratio of soluble protein to total protein.
Other options are incorrect.
Thus, the correct expression is soluble protein/total protein $\times$ 100.
Step 4: Final Answer:
The correct expression is Ripening Index = % soluble protein $\times$ 100 % total protein.
Hence, the correct option is (A).