Punctuation in dependent clause

A dependent clause cannot stand alone; it begins with a word like because, while, although, or when.
When such a clause opens a sentence, put a comma at its end before the independent clause: "While the rain fell, the streets emptied."

A dependent clause is not followed by a colon or a dash.
Those marks belong after an independent clause, so placing one after a dependent clause is an error.
Only a comma links an opening dependent clause to what follows.

One more rule: a comma never comes directly before "that".
In "The study showed that sales rose", no comma belongs before "that".
It reliably introduces its clause with no punctuation in front of it.

So first spot the dependent clause by its opening word.
If it comes first, close it with a comma, and never with a colon or dash.
Getting the order right tells you exactly which punctuation is allowed.

Which choice correctly punctuates the sentence? "Because the bridge was closed ______ we had to take a longer route."
(A) a comma
(B) a colon
"Because the bridge was closed" is a dependent clause coming first, so it needs a comma before the independent clause.
A colon cannot follow a dependent clause, so (A), a comma, is correct.

Which choice correctly punctuates the sentence? "The researchers discovered ______ the ancient tools were older than expected."
(A) no punctuation before "that"
(B) a comma before "that"
A comma never comes directly before the word "that".
So (A), no punctuation, is correct.

Which choice correctly punctuates the sentence? "As the sun set over the hills ______ the campers gathered around the fire."
(A) a comma
(B) a dash
"As the sun set over the hills" is a dependent clause coming first, so a comma is required, and a dash cannot follow a dependent clause.
So (A), a comma, is correct.