4.4. exercise: Fusion preposition and indefinite article
The fusion of de / em with the indefinite article is possible in SPOKEN language on both sides of the atlantic. Concerning the fusion of em + indefinite article in written language there are caveats in Brasil and concerning the fusiion of de + indefinitive articles there are strong caveats in Brasil. In Portugal the fusion of em + indefinite article is the standard and the fusion of de + indefinite article accepted. Summary: The contraction is, apart from the fusion of em + indefinite in Portugal, optional. (This author assumes that originally the fusion was common, because it is still common in Galician, the language Portuguese steems from.)
In this exercise we presume that the fusion is done. Chose the right option.