What is the meaning of 为零 here？
为= being / is

零= zero

Answer D: 比例为零 (proportion is zero) means the proportion of elderly people who can't take care of themselves in the population is zero. (no more elderly people who can't take care of themselves) . That makes no sense. 

The writer of the question might had phrased it wrong, or the answer is just obviously incorrect.

Answer D should had been 比例增减为零(proportion increase or decrease is zero) which mean 'no change'

When 零 means 'wither and fall'(凋零), it only refers to flower petals or leaves, and is mostly used in a poetic sense; When 零 means 'drop and shatter' as in (零落), it is mostly used in a poetic sense also. For example: "tears drop and shatter(涕零)"