During which stage does the birth rate begin to decline?

During stage 3 - late expanding (of demogrpahic transition model)

Explanation:

During stage 3, birth rate begins to decline as infant mortality is lower and women have more access to education, family planning, and contraceptives.  Children are not needed as "free labor" as they might have been in earlier stages.

if the thermos has a vacuum innner, then heat through conv currents and conduction are reduced = vacuum flask

reserve is the answer