It's always a mistake to think of grammar as involving commas and words following them. Grammar is clauses and phrases and predicates; there are no commas in language, only in writing.
In this case, there are two clauses:
- ((Eric's) psychology class) was different from the other classes (that) (Eric) had taken
- (Eric) was unhappy with ((Eric's) psychology class)
The pieces in parentheses are either deletable markers (like relative that) or noun phrases that can be replaced by pronouns (like he, his, and it) in the appropriate circumstances.
The problem is what the appropriate circumstances are for pronominalization, and that's what this question tests.
Pronouns always refer to someone or something that's obvious in context. When the context consists of only one sentence, the word denoting the person or thing (called the "antecedent") must be in the same sentence as the pronoun in order to be obvious.
But not just anywhere in that sentence. As the word antecedent (Latin for 'going before') suggests, normally the antecedent is spoken before the pronoun.
- He likes Eric's psychology class
is a perfectly good sentence, provided he doesn't refer to Eric; otherwise it's garbage. Switch them and it's fine. But this sentence has two clauses: sentence 2 is the main clause, and sentence 1 is a subordinate clause; this makes a difference for pronoun usage.
If the antecedent is in the main clause, and the pronoun is in a clause subordinate to the main clause, then a pronoun can come before its antecedent. For example, consider some simpler sentences:
- Before Marilyn became president I knew her.
- I knew Marilyn before she became president.
- Before she became president I knew Marilyn.
- *I knew her before Marilyn became president.
The first three are fine; in the first two, the antecedent (Marilyn) comes before the pronoun (her or she). In the third, the antecedent is in the main clause but the pronoun is in a subordinate clause, so even though the pronoun precedes its antecedent, it's OK. That's the same structure as the SAT sentence, and that's why the answer says there is no mistake.
But the fourth one is ungrammatical (that's what the asterisk indicates), because the pronoun is in the main clause and it precedes its antecedent, which is in a subordinate clause. So the SAT question tests whether you know the rule that distinguishes the third OK case from the fourth ungrammatical case.
Just going based on what sounds right to my ear:
doesn't -- Preferred if I was just trying to state a simple fact. "13/13/13 doesn't exist". Much as I might say: "Little green men on Mars don't exist."
can't -- Preferred if I was trying to point out how the calendar system prevents such a date. "13/13/13 can't exist because it's an invalid date!"
won't -- Works as long as the date is in the future but sounds a little clunky. Wouldn't make sense to say this about 13/13/13 once 2014 rolls around, even though the date is equally invalid then as it is now.
wouldn't/couldn't -- Sound too wishy-washy, like they're entertaining the possibility that maybe the date could or would exist under the right circumstances.
Given your specific example, I think any of the first three could work, but I'd probably use doesn't.
Best Answer
Since two incorrect answers have now been given to this question (one removed again), I thought the correct answer that Jim already gave in a comment ought to be posted as an answer, too:
‘Its’ here is referring back to the semicolon, not to the comma or the period.
To paraphrase: