Which of the following Binary Min Heap operation has the highest time complexity?

A) Inserting an item under the assumption that the heap has capacity to accommodate one more item

B) Deleting an item from heap

C) Merging with another heap under the assumption that the heap has capacity to accommodate items of other heap

D) Decreasing value of a key

Confused between option C and D. Please explain out of both which is correct and which one is not and why?