Answer:
Therefore
Step-by-step explanation:
Therefore means as a result or consequently. Thus, the transitional word therefore 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 nevertheless and whereas indicate contrast, while rather is usually used to express preference, degree, agreement or acceptance.