A transition which best fills in the gap is,
Clearly, Rainsford knows he has run out of choices and is trapped.
Transitions refer to the shift in sentence or ideas from one to another. Transitional words help in a smooth shifting of ideas without any abrupt change, such as for instance, clearly, additionally and so forth.
The transition words 'clearly' states the idea of clarification regarding the situation. In the above phrase, Rainsford is aware regarding his situation that he is trapped and options. Hence, the transitional words for instance and in addition are used in research and debates.