The core concept here is the impact of changes in male mortality on natural selection. The passage states that the universal pattern of higher male mortality is changing, which means that one of the factors that natural selection operates on is being altered. The correct answer is 'C' because it directly refers to the reduction in opportunities for natural selection due to the change in male mortality. Option 'A' is incorrect because the passage implies an excess of boys in the crucial years. Option 'B' is incorrect because it suggests a balance that the passage does not support. Option 'D' is incorrect because the passage suggests an excess of males, not females, in the reproducing age.