Question

Which part of the iceberg displaces water equal in weight to the buoyant force

Answer 1

The bit that's below the sea level = most of it = part of the reason why the Titanic was "holed below the water line" and thus doomed to sink. Most of a berg is below the water line.

Answer 2

Whoa ! Wait a second. The weight of the displaced water
is ALWAYS equal to the buoyant force.

In the case of a floating object ... whether it's an iceberg or
a dead fly ... the part that's under the surface is the part that's
displacing water.