The correct answer is A) Bluish. Multi-year ice, which is the hardest and oldest sea ice, typically has a bluish tone to its surface color. This is because the ice has been subjected to multiple cycles of melting and refreezing, which compacts the ice and causes air bubbles to be expelled, resulting in a more transparent and bluish appearance. The other options are incorrect because greenish (B) is more characteristic of first-year ice, grey (C) can apply to both multi-year and first-year ice, and grey-white (D) is more common for younger, thinner ice that has not undergone as much weathering and compaction.