If most individuals in a populations are young, why is the population likely to grow rapidly in the future?

The correct option is A Many individuals will begin to reproduce soon

Different age groups have different reproductive capabilities. Pre-reproductive individuals are the young individuals which will enter the reproductive age after some time. They are the potential source of increase in population. Reproductive individuals are the ones which are actually adding new members to the population. Post reproductive individuals are older individuals which no longer take part in reproduction.

So, the correct answer is 'Many individuals will begin to reproduce soon'.