Select the most appropriate option to fill in the blank.
It is only a rope, she ______ herself, though it looked very much like a snake.

The sentence presents a contrast: something looked like a snake (which causes fear), but it was actually just a rope. To remove her own fear or doubt, she would tell herself that it's just a rope.
The word reassured means to say or do something to remove the doubts or fears of someone (including oneself). Thus, "reassured" perfectly fits the context.

retained means kept possession of.

replied means answered.

remembered means brought to mind an awareness of someone or something.