Question

Select the correct form for the verb asistir: Ellen attended a lecture last night. Elena _______ a una conferencia anoche.

Answer 1

The question is saying that Elena attended so it is past tense and because it is referring to Elena it will be el/ella/usted conjugation.

Therefore: it is probably asistió which means attended.
It could be ha asistido though because ha asistido means has attended but Elena has attended a conference does not sound as good as Elena attended a conference.

Answer 2

Right answer: asistió

This a sentence written in Simple Past Tense which is used to express specific actions completed or concluded in the past that are separated from the present.  

In other words:  

It is applied when the person speaking gives information about the past that is not related to the present.  

Now, in Spanish the verbs must be conjugated according the subject and must match in genre and number. Hence, the correct conjugation of the verb asistir (to attend) in Simple Past with the third person in singular she (ella), which is Ellen, is asistió (she attended).

Elena asistió a una conferencia anoche.  

Ellen attended a lecture last night

The other options are incorrect because are not in simple past tense:

asiste (attends) is in present tense

asistiá does not exist as a verb in the spanish language

ha asistido (has attended) is in present perfect tense