💡 Detailed Explanation
The correct answer is 「深刻」 (shinkoku), meaning 'serious', 'grave', or 'severe'. It is used for problems, issues, or situations. 「真剣」 (shinken) means 'earnest/serious' for human attitudes. 「極端」 (kyokutan) means 'extreme'. 「厳格」 (genkaku) means 'strict/rigorous'.