Explanation:

- I am much taller than I was a year ago.

- This sentence is correct and clear as it stands.

- Option 1: "that I were a year ago"

- Incorrect. "That I were" is awkward and incorrect grammatically in this context.

- Option 2: "as I was the year before"

- Incorrect in the specific context but grammatically correct. It changes the meaning by emphasizing the passage of two years.

- Option 3: "No substitution"

- This indicates the original sentence does not need any changes.

- Option 4: "than a year before"

- This option is grammatically correct, but it's simpler to use "than I was a year ago."