Hey Haewon! =)

These explanations should help:

#6

Colon Rule:

The information after the colon must clarify the first part of the sentence.

The information after the colon does not have to clarify the word right before the colon (i.e. I have one goal I want to achieve in my life: to start an online business). As long as you remember this rule, it won't matter whether the information clarifies the word right before the colon or even a phrase or clause that came earlier. In the example sentence, "to start an online business" clarifies the "one goal...I want to achieve in life."

In question #6: the information after the colon clarifies "beaker 124 became murky" since it tells you that the beaker was infested with some kind of organic growth...

#15

Nonessential/essential information can get a bit tricky. You are not alone...here are some things to keep in mind

1. The article "a/an" is only for singular nouns, and that rule only applies to "people"

A businessman, Ed Burns, enrolled in a program that is designed to help people earn more money.

However, in #15, "concrete language elements" is plural, so the article "a" does not apply here; plus, it doesn't refer to a person.

2. Sometimes, information can be both essential and nonessential, especially when it is hard to determine whether the first noun phrase is general or specific.  

concrete language elements, such as reading level and writing speed,

conrete language elements such as reading level and writing speed

It just depends on the writer. One writer might want to treat "such as reading level and writing speed" as essential, while another writer might not. That is why you will sometimes see information that YOU think should be essential being treated as nonessential; for phrases that are hard to determine or could go both ways, it is up to the writer. And trust me, the SAT knows this too. That is why they will NEVER test you on JUST essential vs nonessential when the information could be both. INSTEAD, the SAT is actually testing you on something else...

For example, in #15

Pay attention to the comma after the relative pronoun "which."

which, alongside....

The comma after "which" introduces nonessential information.

Now, where does the "alongside" nonessential phrase end? It has to end at the word "speed," right? If you ignore that nonessential information, you get the relative clause, "which forms the child's capacity for learning another language". That is the correct relative clause.

That is why you have to eliminate (A) and (D), since there is no comma after the word "speed".

On the surface, this question does appear to be testing nonessential/essential with "concrete language elements, such as..." but in reality, it testing you on something else.

It just so happens that the "writer" is treating "such as reading level and writing speed" as nonessential in THIS sentence.

I hope these explanations help! =)

Keep Moving Forward!

You got this!