Complete the sentence with the most appropriate word or phrase out of the given options.
She was wearing a woolen hat and a leather jacket ____ she should feel cold.

lest

The most appropriate word to complete the sentence is lest.

The other options are not as accurate. Lost suggests that she was cold and did not know where she was. Leap suggests that she was about to jump. Let suggests that she was allowing herself to feel cold. Lest is the best option because it suggests that she was wearing the hat and jacket to prevent herself from feeling cold.