Which trait was most important in the development and advancement of modern fish? a) a lower jaw b) eggs with shells c) 4-chambered heart d) live birth of young

A. a lower jaw. hope this helps

Answer: Option A - A lower jaw.

Explanation:

The earlier fishes used to be jawless (Agnathans).The most important trait used in the development of advanced fishes was the development of lower jaw.This lower jaw was primarily developed to facilitate the biting. Later on the complete jaw was developed which facilitated first in breathing and then was utilised for feeding.