The goal of this study is to characterize how pre-service teachers recognize students' reasoning, based in the unitizing process, when students solve ratio comparison problems and, how they consider the unitizing process as a Key Development Understanding to propose teaching decisions. Ninety-one pre-service primary teachers interpreted primary school students' answers to a ratio comparison problem and proposed teaching decisions to improve students' reasoning. Findings indicate that pre-service teachers recognized features of the unitizing process as a Key Development Understanding but had difficulties to propose teaching decisions addressed to improve the students' mathematical reasoning. These findings display the role played by specific mathematical content domain when pre-service teachers make teaching decisions considering the students' mathematical reasoning.