Assuming that A is telling the truth, B should say, "A didn't steal it." C should say, "It's me." Ding should say, "B didn't steal it." What three people said was not contradictory, so C stole it. If you are not at ease, you can continue to reason.

Assuming that B is telling the truth, A should say, "I stole it." C should say, "It's me." Ding should say, "B didn't steal it." What A and C say is contradictory, so it is wrong.

Assuming that C is telling the truth, A should say, "I stole it." B should say, "A didn't steal it." Ding should say, "B didn't steal it." What Party A and Party B said is contradictory, so it is wrong.

Assuming that Ding is telling the truth, A should say, "I stole it." B should say, "A didn't steal it." C should say, "It's me." What A, B, C and D said was contradictory, so the first hypothesis was correct and was stolen by C.