Step 1 - Definition : Infant Mortality Rate (IMR) indicates the number of children that die before the age of ONE year as a proportion of 1000 live children born in that particular year.
Step 2 - Formula :
$$IMR = \frac{\text{Number of deaths of children below 1 year}}{\text{Total live births}} \times 1000$$
Step 3 - Hence the blank is filled by 'One'.

Answer: (A) One