Right answer:
entró
This answer makes sense because entró comes from the verb entrar and is the conjugation of this verb for the third person singular in the preterite tense. This tense is used to describe actions that took place or were completed at a certain point in the past. So he entered Mr. Gomez's office at a certain moment in the past. Finally:
IN SPANISH:
Después, él entró en la oficina del señor Gomez.
IN ENGLISH:
Afterwards, he entered Mr. Gomez's office.