¡Cuidado! ¡No ..... caigas!
B
Correct answer
Explanation
The blank requires a reflexive pronoun that matches the verb caigas (second person singular subjunctive of caerse). The negative command No te caigas means Dont fall, using te as the correct pronoun for tu (you). Se is third person, ti is a prepositional pronoun not used before verbs, and tu is a subject pronoun.