The correct answer is option 'D' - 30 MPa for post-tensioned work and 40 MPa for pretension work.

Explanation:

1. Limitations of IS 1343-1980:
IS 1343-1980 is the Indian standard code of practice for prestressed concrete. This code sets the minimum requirements for the design and construction of prestressed concrete structures. It includes provisions for both post-tensioned and pretensioned work.

2. Minimum characteristic strength for post-tensioned work:
According to IS 1343-1980, the minimum characteristic strength of prestressed concrete for post-tensioned work is 30 MPa.

3. Minimum characteristic strength for pretensioned work:
According to the same code, the minimum characteristic strength of prestressed concrete for pretensioned work is 40 MPa.

4. Difference between post-tensioned and pretensioned work:
Post-tensioned work refers to the process of tensioning the prestressing tendons after the concrete has hardened, while pretensioned work refers to the process of tensioning the tendons before the concrete is cast.

5. Reason for higher strength requirement in pretensioned work:
The reason for the higher minimum characteristic strength requirement in pretensioned work (40 MPa) compared to post-tensioned work (30 MPa) is that in pretensioning, the tendons are tensioned before the concrete is cast. This requires higher strength concrete to withstand the high initial tensioning forces applied to the tendons.

6. Importance of minimum characteristic strength:
The minimum characteristic strength is set to ensure the structural integrity and safety of prestressed concrete structures. It provides a minimum level of quality and strength that the concrete must possess to perform as intended.

In conclusion, according to IS 1343-1980, the minimum characteristic strength of prestressed concrete for post-tensioned work is 30 MPa, while for pretensioned work, it is 40 MPa. These requirements are specified to ensure the structural integrity and safety of prestressed concrete structures.