Step 1: Understanding the Statement I:
Photochemical smog is also known as Oxidising Smog. It occurs in warm, dry, and sunny climates and contains oxidising agents like Ozone (\(\text{O}_3\)) and Peroxyacetyl nitrate (PAN).
\(\rightarrow\) Statement I is Correct.
Step 2: Understanding the Statement II:
Classical smog (London smog) occurs in cool, humid climates and is chemically reducing in nature. It consists of smoke, fog, and sulphur dioxide (\(\text{SO}_2\)). Nitrogen dioxide (\(\text{NO}_2\)) is a primary component of Photochemical smog, not classical smog.
\(\rightarrow\) Statement II is Incorrect.
Step 3: Understanding the Statement III:
High concentrations of \(\text{SO}_2\) affect plants. It causes chlorosis and specifically leads to the stiffness of flower buds, which eventually fall off from plants.
\(\rightarrow\) Statement III is Correct.
Step 4: Understanding the Statement IV:
Normal rain water has a pH of about 5.6 due to the dissolution of atmospheric \(\text{CO}_2\) forming carbonic acid. pH 7.5 would be alkaline, which rain water is typically not. Acid rain has pH<5.6.
\(\rightarrow\) Statement IV is Incorrect.
Conclusion: Statements I and III are correct.
Required Answer: Option (A).