Hola Mollie

The auxiliary verb haber is used to form the present perfect tense of other verbs, i.e. to say I have gone, you have done, he has worked, etc.. 

The first part of the present perfect structure is the verb haber in the present, as in saying: I have..., you have..., he has... and then the second part is the main verb in the past participle form: ido, trabajado, tenido...etc. Here, in this lesson we are only testing the first part of the verbal structure, the part with haber: he, has, ha, hemos, habéis, han. 

So here in this sentence you need:

Yo he llegado pronto a trabajar esta tarde.

I hope this clarified it.

Saludos

Inma