Step 1: Concept
Prepositions indicate relationships between nouns. The preposition "of" is frequently used to indicate measurement, value, or quantity.
Step 2: Meaning
The sentence describes the measurement or extent of the coastal line.
Step 3: Analysis
To connect the noun "costal line" with its specific measurement "960 KMs," the preposition "of" is the standard choice.
Step 4: Conclusion
The correct preposition is "of," making the sentence: "There is a useful costal line of 960 KMs in our state".
Final Answer: (C)