<em>The rule that applies to the word desire is </em><em>Group II. When the root is a full word except for a final e, the forms usually are spelled with -able.</em>
Explanation:
Suffix <u>-able</u> is used to turn verbs into adjectives and means that something can be done or that is capable of something. There are many spelling rules used to introduce the suffix <u>-able; </u>the one that applies to the word <em>desire </em>is Group II, since it turns the word <em>desire </em>into <em>desirable</em>.