Answer:
a. To communicate how rare it is to find the bloom.
Step-by-step explanation:
In the given passage, the author talks about how the yucca tree may be useless and "its woody skeleton, with hardly power to rot, makes even the moonlight fearful." But irrespective of how the tree may be, there is one good thing about the plant. It produces the best bloom which is "luxurious, creamy, cone shaped bud of the size of a small cabbage, full of sugary sap". This is what the Indians used to "roast the prize for their own delectation", which shows that the bloom is something that they treasure, something precious and worth more than it seems to be. The author's use of the word "prize" shows that he bloom is rare and hard to find/ get, laying emphasis on the bloom despite the hardship it entails in extracting it from the "fence of daggers".