Answer:
The Direct Object, is the part of the sentence that receives the action of the verb. It is used to replace a person, animal or thing in the conversation or in the writing in order to avoid repetition.
According to the Subject pronouns the Direct Objects are:
1st person singular: Yo me
2nd person singular (informal): Tú te
2nd person singular (formal): Usted lo/la
3rd person singular: El/eso lo, Ella/esa la
1st person plural: Nosotros nos
2nd person plural: Ustedes los/las (In latinAmerica)/Vosotros os (In Spain)
3rd person plural: Ellos/Ellas los/las
In the sentence:
Ayer (Yo) compré el jabón
Yesterday I bought the soap
El jabón (the soap) is the part of the sentence that receives the action of the verb comprar (to buy), which in this case is conjugated in past tense compré (I bought).
In addition, El jabón, is identified as the 3rd person singular as eso (it) in masculine pronoun.
Hence, the Direct Object that correctly matches with El jabón is lo.
Finally the correct answer is C:
