<em>Therefore </em>means as a result or consequently. Thus, the transitional word <em>therefore </em>is necessary because the second sentence introduces an outcome, an effect or repercussion of the first sentence. In this manner, the fact that Bigfoot sightings are not taken seriously causes no significant study to take place.
The rest of the options are incorrect because <em>nevertheless </em>and <em>whereas </em>indicate contrast, while <em>rather </em>is usually used to express preference, degree, agreement or acceptance.
No one takes these Bigfoot sightings seriously therefore, no significant scientific study of either the facts or the cause of the sightings has been done.
No significant scientific study of either the facts or the cause of the sightings has been done therefore, no one takes these Bigfoot sightings seriously.
