Right answer: Acabo de trabajar mucho
The verb acabar (to finish) means to complete an activity, to accomplish it to the end. Conjugated in present tense (indicative) with the 1st person in singular yo (I) is acabo ( I finished).
Now, this verb is also used as a verbal periphrasis or verbal phrase to mean that somebody has just done or finished something. This is with the rule below, as follows:
Subject + acabar (in its many conjugations) + de + main verb in infinitive + complement
Note in spanish sentences it is not always necessary the subject.
In the case of the sentence:
(Yo) Trabajo mucho (I work a lot)
The verb trabajar (to work) is in present tense with the 1st person in singular.
So, if we change this sentence to the verbal periphrasis form shown above, taking into account the verbal tense we have:
(Yo) Acabo de trabajar mucho
I just worked a lot