Answer:
a. tuvo
Step-by-step explanation:
In Spanish, the preterite tense is used to talk about completed actions or events in the past. The preterite conjugation of the verb "tener" (to have) is "tuvo" for the first person singular (yo) form. So, the correct sentence would be "Se dio cuenta que tuvo reacciones más rápidas." This means "He/she realized that he/she had faster reactions."