Fill in the blank with the most appropriate word.

There is a ______ growth of flowers in tropical countries.

luxuriant

The most appropriate word to fill in the blank is luxuriant.

The word "luxuriant" means growing thickly and profusely. It is the most appropriate word to use in this context, as it conveys the meaning of the growth of flowers being very abundant and healthy.

The other options are not as appropriate.

• Pious means having or showing a dutiful regard for religious beliefs and practices. This is not the right word to use, as it does not make sense to say that the growth of flowers is pious.
• Plenty means a large or sufficient amount. This is not the right word to use, as it does not convey the same meaning as "luxuriant."
• Powerful means having great strength or force. This is not the right word to use, as it does not make sense to say that the growth of flowers is powerful.