Answer:
The correct answer is option D. en la segunda hora.
Explanation:
Of the options given, there are only two that are grammatically well written:
<em>"En la sexta hora"</em> and <em>"En la segunda hora."</em>
Remember that articles must match in gender and number, which does not happen in options A and C.
Of the two remaining options, the correct one is “en la segunda hora”, since the other would be translated as “in the sixth period”.
In Spanish it is not divided into periods but into hours, therefore the correct answer is option D.
