For $7$ nodes, minimum height = $2$.
Options B and D give fractional heights. So, they’re wrong.
Options A and C give $3$.
So, the question assumes the root to be at height $1$.
Rewriting: For $7$ nodes, maximum height = $3$.
For $6$ nodes, it will still be $3$.
Only Option A satisfies this.
The answer is also Option A in the official answer key.
Edit: Meant to comment on the question, lol. Not on this answer.