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.