Answer:
D. palace
Step-by-step explanation:
Riverboats is defined as the boats used for transportation purpose on rivers, lakes, and artificial waterways. Riverboats were used for carrying trades, for people transport, and as tour boats.
Riverboats were like palace because it was built in such a way that it can be used for tour purpose for more than one day and include luxury units in a boat such as accommodation, food service and entertainment same as a palace.
Hence, the correct answer is "D. palace".