Answer:
The correctly punctuated conjunctive adverb can be found in option c. ; however,
The sentence is then:
These obvious contamination problems have long been known; however, what is not often realised is the organic matter carried in ground water can contaminate samples.
Step-by-step explanation:
"However" indicates a relationship of contrast between two independent clauses. When we wish to join those clauses to form a compound sentence, we must punctuate properly. The right way to do it is by placing a semicolon before and a comma after "however". This is why letter C is correct.
Another possibility would be using a full stop/period before "however" and a comma after it. In this case, we would not be forming a compound sentence, though. Letter D is incorrect because it is missing both the period and the comma. Letters A and B do not follow any of those possibilities.