The first option is the answer: <em>Titinius feels guilty that Cassius is dead, and decides he must kill himself.</em>
How I built the conclusion:
Firstly, the act starts with a conversation between Messala and Titinius. Then, Messala EXITS, so a monologue by Titinius starts (Since he's the only one in the scene, there's no need to identify the speaker, as there was previously when Messala was also present).
Since he's alone, and speaking with a sorrow tone when he starts to 'talk' to Cassius, it can be assumed that Cassius is dead. And apparently, Brutus had previously instructed Titinius to give a garland (the <em>wreath of victory</em>) to Cassius. (The notice of Cassius death is probably the report that would <em>thrust Brutus' ears</em>, and what Messala left to tell him, in the first place). Finally, he decides he will still accomplish his mission (<em>Brutus' bidding</em>) by also dying (as giving Cassius his garland in the afterlife).
So with the <em>gods' leave</em> (in this context, permission), he meets his <em>Roman's part</em> (separation from Rome - material world). And he finally conclude the deed by 'inviting' <em>Cassius' sword </em>to find <em>Titinius' heart </em>(his own heart - thrusts a sword into his chest).
