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Question #: 1: It is very easy.  Is the answer incorrect because the word 'very' modifies 'is', and is therefore an adverb, and not 'easy' which is an adjective? -- No.  Adverbs modify verbs and adjectives.  'Very' is an adverb modifying the adjective 'easy.

Question #: 5: It was likely to happen.  How is that 'likely' does not modify the infinitive verb 'to happen' and therefore be an adverb? How is it it possible that likely is an adjective? -- 'Likely' is an adjective modifying 'it'.  'Be' is a linking verb.  'To happen' is an infinitive clause complement.

Second, why does the following sentence not have a comma after the quotation: "Ouch!" yelled Cecilia. "You're stepping on my toes." Shouldn't there always be a comma after a direct quote involving reported speech?-- Not if the sentence ends.

Third, why does the following sentence have a subject-verb agreement error: "This is some of the data that shows why he is wrong." -- 'Data' is a word which is currently of unsure number.  If the writer conceives of data as a single accumulation, then s/he will use a singular verb.

Fourth, why is the relative pronoun their incorrect in this sentence: "Did everyone bring their permission slip?"-- It is not a relative pronoun, it is a possessive adjective, and it is not really incorrect, though it remains 'incorrect' for some English tests, which may require the singular 'his' (or 'her'!).


Finally, what's the rule for using the possessive pronouns its versus their when referring to plural nouns? For example, the team lost (its/their) game last nite or the committee/crowd/organization/company lost (its/their)..whatever it may be. -- Both are acceptable here.  BrE tends to use one while AmE tends to use the other (and I've forgotten which is which), but ultimately it lies in the mind of the speaker-- whether s/he is conceiving of a single group or several united individuals.  In sone cases, a choice is forced:  'the commitee have been arguing among themselves for several hours.'