Question: What is the \'union by rank\' optimization in Disjoint Set Union?

Options:

1. Always attaching the smaller tree under the larger tree
2. Always attaching the larger tree under the smaller tree
3. Randomly attaching trees
4. Merging trees based on their height

Correct Answer: Always attaching the smaller tree under the larger tree

Solution:

\'Union by rank\' optimization involves always attaching the smaller tree under the larger tree to keep the overall tree height minimized.