Step 1: Correcting sentence (i).
The word "preferable" is a comparative adjective, and when comparing two things, the correct preposition to use is "to," not "than." The corrected sentence should compare milk and tea using "to."
Step 2: Corrected sentence (i):
Milk is preferable to tea.
Step 3: Conclusion.
The corrected sentence is:
Milk is preferable to tea.