Answers:
The Direct Object, is the part of the sentence that directly 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; when they have already been seen in a sentence or external context where the statement occurs.
According to the Personal Pronouns the Direct Object Pronouns 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(In latinAmerica) los/las/Vosotros(In Spain) os
3rd person plural: Ellos/Ellas los/las
Knowing this, let’s begin with the answers in this paragraph that narrates the brief encounter of two people (Sofía and Armando) who were classmates and do not recognize each other:
Él no (1) la saluda
Before this sentence there is another giving the information about the genre of the subject Armando does not say hello, referring to Sofía who is the 3rd person in singular she (ella in spanish). As we could see from the list above the corresponding direct object pronoun is la
porque no (2) la reconoce
This is the same case, because it is being talked about Sofía.
… y Sofía (3) los terminó en 2002.
Here we are referring to the studies (los estudios) which is a plural masculine noun related to the 1st person in plural they, therefore the direct object pronoun is los
De hecho, Armando nunca (4) los llama
Here we are referring to los compañeros (the classmates), this is also a plural masculine noun related to the 1st person in plural they, hence the direct object pronoun is los
Sólo habló por teléfono con Sofía una vez porque ella (5) lo llamó.
In this sentence we are referring to Sofía who once called Armando (him). Armando is the 3rd person in singular he and the corresponding direct object pronoun is lo