It's simple
when 2 people marry , they have kids
those kids have their kids
then their kids and so on
so population increases
more detailed
Since most fertility occurs in marriage, a rise in the age of marriage can reduce world fertility. ... Later marriage means less female fertility exposed to intercourse, which can particularly affect already subfecund women. The number of women in professions and careers also dampens fertility rates.
D. HUEY P. LONG I KNOW THE ANSWER IS D BECAUSE I HAD TO ANSWER THE SAME THING