Is Venice built on poles, piles, or pilings?

Which term should be used to describe the wooden support system of Venice?

Piles is the most commonly used term (wooden piles, to be exact), but poles would also work, as does pilings. Stilts, which isn't one of your options, is an even more precise descriptor:

stilt
  • each of a set of posts or piles supporting a building above the ground.

The Oxford American Writers’ Thesaurus has even more synonyms for pile in this meaning:

pile (2. a heavy beam or post driven vertically into the bed of a river, soft ground, etc., to support the foundations of a structure; a wall supported by timber piles):
  • post, stake, pillar, column, support, foundation, piling, abutment, pier, cutwater, buttress, stanchion, upright.

Piles has a specific civil engineer meaning. They are foundation pillars that are driven into the ground - as opposed to columns that are placed on a foundation.

Pilings and piles are mostly interchangeable, pile refers to the actual object, piling to the whole system and the process. So you have a 'pile driver' but a 'piling foundation'.