In the fun function:

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int fun(int i) { return i++; }

The post-increment operator i++ returns the current value of i and then increments it. So, fun(i) will return the current value of i (which is 1) and then increment i to 2.

In the main function:

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int i = 1; i = fun(i); printf("%d", i);

Here, i is assigned the result of fun(i), which is 1. So, the program prints the value of i, which is 1.

Therefore, the correct answer is D. The program outputs 1.