To solve this question, the user needs to have knowledge of software testing concepts, specifically testing related to data conversion from existing systems to new systems.

Option A: Data driven testing is a type of testing where test input or output values are read from data files instead of using hardcoded values. This type of testing is not directly related to testing software used to convert data from existing systems to replacement systems.

Option B: Configuration testing is a type of testing that checks whether the software is properly configured according to the requirements. This type of testing is not directly related to testing software used for data conversion.

Option C: Migration testing is a type of testing that verifies the data and software functionality after migrating from the old system to the new system. This type of testing is directly related to testing software used to convert data from existing systems for use in replacement systems. Thus, option C is the correct answer.

Option D: Back to back testing is a type of testing where two identical systems are set up, and transactions are processed on both systems simultaneously to compare the results. This type of testing is not directly related to testing software used for data conversion.

Therefore, the answer is: C. Migration testing.