Question

Group I. If the root is a full word , the ending is usually -able.

Answer 1

The answer:  " -ible"

The word is:  "exhaustible" .

Group:  VII .
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The following rule applies, as mentioned within the question, and as quotated DIRECTLY from THIS VERY QUESTION:

"Group VII. When the word has an immediate -tion form, -ible is used." 
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 Note:  The word, "exhaust" ; does, in fact have an "immediate -tion form" ; 

since the word, "exhaust" ;  ends with the letter, "t" ; and since the word, "exhaust" ; has an "immediate -tion" form, which is the word, "exhaustion" ;  then "Group VII"  applies.
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The word is "exhaustible" ;  (note that the ending is:  "-ible"). 

Group:  VII .  
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