“Improvement” where used in “special improvement district” means any public improvement. It shall be liberally construed to include:

1. (a) Any acquisition of land, rights of way, or easements for public use.
2. (b) Any acquisition or construction of buildings, structures, or public works.
3. (c) A combination of any or all of the foregoing.
4. (d) All acquisitions of land and all acquisition and construction of public works authorized by law.