What ancient custom involved placing the deceased in a boat and cremating it with necessary items?

The correct answer to the question regarding the ancient custom of placing the deceased in a boat and cremating it with necessary items is indicative of a practice known as Water Burial. In this custom, the deceased is often placed in a vessel or boat, which is then burned as part of the funeral rite, symbolizing the journey to the afterlife and the release of the spirit. This method finds roots in various cultures around the world, highlighting the significance of water as both a boundary and a means of spiritual transition.

Mound burial typically refers to interment practices involving the construction of burial mounds but does not involve cremation or the use of boats. Sky burial is associated with certain Tibetan practices where the deceased is placed on a mountaintop for exposure to the elements and scavenging birds, while earth burial refers simply to the burial of the deceased in the ground without the elements of cremation or the use of water as described in the context of this question. Thus, Water Burial aptly captures the essence of the described ancient rite.