For calculation of incidence denominator is taken as: 
a. Midyear population 
b. Population at risk c. Total number of cases 
d. Total number of deaths 

The denominator for the calculation of incidence refers to the total number of individuals who are at risk of developing a particular condition or disease. It is important to have an accurate denominator because it provides a basis for comparison and helps in determining the frequency or risk of the disease in a population.

The choice of denominator depends on the specific situation and the purpose of the study. Let's understand the options provided:

a. Midyear population: This refers to the population estimate taken at the midpoint of a specific time period. It is commonly used in population-based studies to represent the size of the population that is at risk during that period.

b. Population at risk: This refers to the subset of the total population that is susceptible to developing a particular disease or condition. For example, if you are studying the incidence of a specific disease among adults, the population at risk would be limited to only adults, not children or elderly individuals.

c. Total number of cases: This refers to the individuals who have been diagnosed with a specific disease or condition. Using this as the denominator would not be appropriate for calculating the incidence, as it does not consider individuals who have not been diagnosed, but are still at risk.

d. Total number of deaths: This refers to the individuals who have died due to a specific cause. Similarly to the previous option, using this as the denominator would exclude individuals who are at risk but have not died.

In summary, the most appropriate denominator for calculating the incidence would be the population at risk, as it considers all individuals who are susceptible to the disease or condition being studied.