About use the word "Insubstantial"

[Zishuli]

Lets see this question in Test #GRE / M34:

7. adj. insubstantial

(a) highly imaginative but unlikely
(b) containing seeds of later development
(c) lacking in nutritive value
(d) of or relating to or involved the practice of aiding the memory; mnemotechnical

The given answer is (c) "lacking in nutritive value". I looked up the dictionary and found exactly this definition. However, I wonder if option (a) highly imaginative but unlikely can possibly be accepted.

[Gray]

Oh, that insubstantial thing led me here --

"And, like this insubstantial pageant faded,
Leave not a rack behind. We are such stuff
As dreams are made on, and our little life
Is rounded with a sleep."
--The Tempest, Act IV, Scene 1, William Shakespeare, 1611

What did he mean by insubstantial pageant?
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First lesson - English, not english. I, not i. ~A student of English