What rule applies to this word?
exhaust
Group IV. When the root has other forms built on the letter a, the -able form is used.
Group VII. When the word has an immediate -tion form, -ible is used.
Group V. When the root ends in hard cor hard g. -able is used.
Group II. When the root is a full word except for a final e, the forms usually are spelled with -able.
Group VI. When the root is not a full word, -ible is used.
Group VIII. When the root ends in ns, miss, or soft cor gible is used.
Group I. If the root is a full word, the ending is usually -able.
Group III. The suffix -able always follows i.