The number of deaths during a year compared with the total number of persons exposed in the class is known as the

Morbidity rate refers to the incidence of disease or health issues within a population, rather than the number of deaths. It measures how many individuals are affected by a particular health condition, which does not provide information about mortality or death rates. Therefore, it is not applicable to the context of comparing deaths to the exposed population.

The retention ratio indicates the proportion of individuals who remain in a study or program over time, rather than measuring deaths. It does not relate to mortality or the fatality of conditions within a population, making it irrelevant to the question at hand.

Permissible loss ratio is a concept used primarily in risk management and insurance, expressing acceptable levels of loss compared to the total possible exposure. It does not pertain to the measurement of deaths in a population and therefore is not applicable in this context.