Which DATA step will give the last observation from the data set called dogs?

The first option is the correct choice because it effectively utilizes the END statement, which is a powerful feature in SAS that allows the programmer to determine when the last observation of a dataset has been processed. By using the END statement with a temporary variable named "last," the DATA step can check if the current observation being processed is indeed the last one in the dataset. Consequently, any logic applied within that DATA step could be tailored to execute specific operations when this last observation is encountered.

The other options do not effectively pinpoint the last observation in the dataset. For example, the second option uses a BY statement with an assumption that the data is sorted by "Breed." Although this could be useful for processing groups of data, it does not specifically yield the last observation of the entire dataset. The third option simply assigns the value 2 to a variable "Obsnum," which does not reference the last observation of the dataset at all. Therefore, these alternatives do not fulfill the requirement of efficiently identifying or extracting the last record from the dataset called dogs.