Miguel fell to the floor in the past, which means that the eating also happened in the past. Therefore Horacio y Fernando ate in the past - we need a past tense.
almorzábamos is for first, not second person plural, so it's ungramatical. <span>almuerzan is present tense.
From the options, the best one is </span><span>almorzaron, but actually you should rather use a continuous tense - almorzaban. </span>
The second one "<span>Horacio y Fernando almorzaron en la cafetería cuando Miguel se cayó al suelo." because <em>almorzaron</em> is past (and plural), and <em>cayó</em> is also past.</span>
