What rule applies to this word? desire group i. if the root is a full word, the ending is usually -able. group ii. when the root is a full word except for a final e, the forms usually are spelled with -able. group iii. the suffix -able always follows i. group iv. when the root has other forms built on the letter a, the -able form is used. group v. when the root ends in hard c or hard g, -able is used. group vi. when the root is not a full word,-ible is used. group vii. when the word has an immediate -tion form, -ible is used. group viii. when the root ends in ns, miss, or soft c or g, ible is used.

The correct answer is "Group II. When the root is a full word except for a final e, the forms usually are spelled with -able"

Explanation:

The passage presented in the question explains the basic rules for adding the suffix "-able" to a root word or basic unit that forms words, for doing this, there are eight different rules that define the correct way to add this suffix mainly based on the features of a word. In the case of the word "desire" this word is a root by itself which means it cannot be reduced to smaller particles and this word ends in "-e", also the correct form of adding the suffix "-able" is in the form "desirable" which means the final "e-" is omitted and the suffix is added. Therefore, this word belongs to the rule "Group II", because as desire words of this group are full words excerpt by the "-e" that need to be omitted to add the prefix.