Final answer:
The IN= option in the MERGE statement of SAS is used to identify data set contributors by creating a variable that specifies whether the observation is from a particular data set, useful for identifying matches and the source of merged data.
Step-by-step explanation:
The option that can be added to the MERGE statement to help identify data set contributors, and thus identify the matches, is a. IN=. When merging multiple data sets in SAS, the IN= data set option is used to create a variable that indicates whether the observation is from a specified data set.
This option is particularly useful when you need to determine the source of each observation after the merge, allowing you to identify which observations are present in both data sets (matched), only in one data set, or if there were non-matching observations in the other. Option b. OBS= limits the number of observations read from a data set, c. RENAME= allows you to rename variables, and d. MATCHES= is not a valid SAS data set option for the MERGE statement.