Which is the best correction of the clause fragment?
since they had traveled often.
a.
delete the word "since."
b.
add a comma after "since."
c.
change "they" to "richard and denise."
d.
change "since" to "because."

Well,

Let's test the options, starting with option D.

"Because they had traveled often."  This still is a dependent clause, and does not make sense.

Option C:
"Since Richard and Denise had traveled often." This is also a dependent clause.

Option B:
"Since, they had traveled often."  This makes even less sense.

Option A:
"They had traveled often."  Here we finally have an independent clause that can stand as a sentence by itself.  This is the correct option.