The correct answer is: C. 42 MPa and 53 MPa.
The cube strength of controlled concrete to be used for pretensioned and post-tensioned work respectively should not be less than 42 MPa and 53 MPa. This is according to the Indian Standard Code of Practice for Prestressed Concrete (IS: 1343-2002).
The cube strength of concrete is a measure of its compressive strength. It is determined by testing a concrete cube of 150 mm side under compression. The minimum cube strength required for different types of concrete is specified in the relevant codes of practice.
The higher cube strength required for post-tensioned concrete is due to the fact that post-tensioned concrete members are subjected to higher stresses than pre-tensioned concrete members. The higher stresses in post-tensioned concrete members require a higher cube strength to ensure that the concrete does not fail under load.