Step 1: Question Analysis.
The query requires identification of the country from the provided list that was not a member of NATO as of December 31, 2024.
Step 2: Option Evaluation.
- Option A: Poland is confirmed as a NATO member. - Option B: Correct. Ukraine, despite ongoing discussions regarding NATO accession, had not attained membership by December 31, 2024. - Option C: Lithuania is confirmed as a NATO member. - Option D: Italy is confirmed as a NATO member. - Option E: Turkey is confirmed as a NATO member.
Step 3: Determination.
The definitive answer is Ukraine.
Final Answer: \[\boxed{\text{B}}\]