Visby is the largest city on the Swedish island of Gotland; arguably the best-preserved medieval town in Scandinavia, and has become a World Heritage Site.

(A) arguably the best-preserved medieval town in Scandinavia, and has become a World Heritage Site.
(B) it is arguably the best-preserved medieval town in Scandinavia, and, as a result of this, has become a World Heritage Site.
(C) arguably the best-preserved medieval town in Scandinavia, it has become a World Heritage Site.
(D) it has become a World Heritage Site, and arguably the best-preserved medieval town in Scandinavia.
(E) is arguably the best-preserved medieval town in Scandinavia, a World Heritage Site.

The issues:

We have IC + Semicolon + Something. . This something should be another independent clause. We have the following two options to fix the problem:

1. We need just another IC.
2. We need conjunctive adverb + COMMA + IC.

Now, we can search the options.

A. There is no subject. It is not an IC.
B. It has the subject and verb, but it has another problem.

it is arguably the best-preserved medieval town in Scandinavia, and, as a result of this, has become a World Heritage Site.

If we eliminate the non-essential part, we get:

it is arguably the best-preserved medieval town in Scandinavia, and has become a World Heritage Site.

==> COMMA + AND is not correct in this context. When we use two verbs for a subject, we should not use COMMA + AND. We just need to use AND to join two verbs.

Example:

I have done the work and won the first prize. --Correct.
I have done the work, and won the first prize. ---Incorrect.

C. C is grammatically correct because "arguably..............., it...." is okay. Arguably........... part has modified IT, and we know IT refers to VISBY.

D. It has the same problem as it is in B.

E. The verb "is" does not have a subject.

Answer is C.