Final answer:
Voldemort drinks unicorn blood to sustain his life until he can regain his full power, which Firenze explains is akin to a half-life, a cursed existence.
Step-by-step explanation:
According to Firenze, a character in the Harry Potter series, Voldemort drank unicorn blood for a very specific and dark purpose. In 'Harry Potter and the Philosopher's Stone' (titled 'Harry Potter and the Sorcerer's Stone' in the U.S.), Firenze tells Harry that the reason anyone would drink unicorn blood is that it can keep you alive even if you are an inch from death. Voldemort, who at this point in the series is weakened and not fully restored to his body, drinks unicorn blood to sustain his own life, albeit as a cursed half-life, as the act of killing something as pure as a unicorn is considered a grave sin.
The correct answer to why Voldemort drank unicorn blood is A. To gain immortality, which in his case translates to a desperate attempt to remain tethered to life until he can regain a full body and power.