Final answer:
Upon finding a major floodplain regulation violation, you can issue a warning, impose fines, require compliance adjustments, or take legal action against the builder.
Step-by-step explanation:
If the third inspection reveals that the builder of an apartment house has made a change to the structure resulting in a major violation of floodplain regulations, several actions can be taken. First, you can issue a warning to the builder, which serves as an official notice of the infraction. Second, fines may be imposed to penalize the non-compliance.
Third, you have the authority to require the builder to make necessary changes to comply with the regulations. Lastly, if these measures are insufficient or the builder does not comply, you can take legal action against him or her.