Answer:
The answer seems indeed to be letter A. Despite living farther away from my family now, I make a greater effort to see them, which is why I visit at least once a month.
Step-by-step explanation:
We are supposed to combine the different sentences and form a logical complex sentence. A complex sentence is formed by joining an independent clause and a dependent one. Notice that letter B does not make the sentence "I visit at least once a month" into a dependent clause. All it does is use a semicolon to attach it to the other sentence.
Letter A, on the other hand, is more effective in connecting the sentences by adding "which is why." That also helps create a more logical sentence, since "which is why" establishes a cause-and-effect relationship between the ideas.