First, let give the bits some names, I put them inside the parentheses:
Petal 1 (p1) = II (c2) + III (c3) + IV (c4)
Petal 2 (p2) = I (c1) + III (c3) + IV (c4)
Petal 3 (p3) = I (c1) + II (c2) + IV (c4)

As p1, p2 and p3 are created from c1, c2, c3, c4, we check each change in c1, c2, c3, c4:
When c1 change: p2 and p3 won't match the value calculated using the formular above (i.e: c1 + c3 + c4 != p2 and c1 + c2 + c4 != p3)
When c2 change: p1 and p3 will go wrong
When c3 change: p1 and p2 will go wrong
As c4 contribute to the value of all three petals, if it is changed then p1, p2 and p3 will all go wrong.

In short:
Calculate ep1, ep2 and ep3 from the first 4 bits (the e in ep stand for expected), then compare them with the given p1, p2 and p3:
    All three pairs do not match: c4 is changed
    p1 and p2 do not match:       c3 is changed
    p1 and p3 do not match:       c2 is changed
    p2 and p3 do not match:       c1 is changed
    Only p1 not match:            p1 is changed
    Only p2 not match:            p2 is changed
    Only p3 not match:            p3 is changed