Since “la semana” is when you say “the first week” the word is written in the feminine form.

feminine, primera
feminine, primero
masculine, primera
masculine, primero

The right option that fills the blank is:

- Since “la semana” is femenine, when you say “the first week” the word primera is written in the feminine form.

Explanation:

A simple way to determine if a noun is masculine or feminine is to identify that feature in the given article that accompanies it (if it has one) since each of these four articles applies to a specific gender and number as sample below:

Since the mentioned noun is "la semana", it can be recognized that the noun is feminine and plural, taking this into account it must be recognized that the adjectives must also bear these traits, that is, in this case, the adjective "primero", must be in its feminine and singular form:

The reason why is selected the form "primera".

i think the answer would be idk