In which trimester does the following occur? the respiratory system becomes mature. first second third

It's definitely the third trimester.

Answer is third trimester.

The development of respiratory system starts early in the fetus. It begins at the 4 week of pregnancy. The respiratory system matures by week 28 (third trimester). By that time  enough alveoli get matured so the even baby delivered immature, can breathe its own. However, respiratory system fully matured during early childhood.  

what are the choices?

again, you should probably take him to the vet. goodluck