Un, an easy way to tell is through the ending vowel.
The 'o' in boligrafo is masculine, so the there are two automatic answers brought forward, 'un', and 'el'.
Then, in english, it would be like saying 'the' or 'a' whereas the is either 'el' or 'la', while a a 'un' or 'una' respectively.
Answer:
Ellos pusieron los postres en la mesa.
Explanation:
I know this because I know Spanish and all my life I speak Spanish, what I don't know is English.
The correct answer is a. el,la
To fill in the blank put the word son.