If fertility has declined and the child share is falling, but total school enrolment continues to rise for some years, which explanation is most appropriate?

Correct answer: Large cohorts born earlier may still be passing through school ages

A decline in fertility affects the number of babies being born now, but it does not immediately remove children who were born in earlier years. Population has age structure, so different birth cohorts move through school ages at different times. If a relatively large cohort was born before fertility declined, many of those children may still be entering or attending school. Their presence can keep total enrolment rising for several years, even while the proportion of children in the whole population begins to fall.

Option A correctly explains this time lag between lower births and school demand. Option B is the opposite of the demographic effect. Option C ignores the direct connection between the size of school-age cohorts and enrolment. Option D is implausible and unsupported, since elderly people do not replace children in ordinary school-age enrolment. Thus, A is the most appropriate explanation.