Final answer:
The two large cylinders that cause an erection in the phallus are known as the corpora cavernosa, which fill with blood during sexual arousal (option a).
Step-by-step explanation:
The two large cylinders that cause an erection are called the corpora cavernosa. These are two columns of erectile tissue that run side by side along the top of the shaft of the phallus. When sexual arousal occurs, these tissues fill with blood, expanding the size of the phallus and making it erect. The corpus spongiosum is another column of erectile tissue that runs along the bottom of the shaft, contains the urethra, and helps maintain the shape of the erect phallus but is not primarily responsible for the erection's rigidity.
Unlike the corpora cavernosa, the corpus spongiosum does not constrict the outflow of blood to the same extent, allowing for semen to be ejaculated. The bulbourethral glands and penile shaft vessels referred to in the multiple-choice options are not the main erectile tissues responsible for an erection.
Hence, the answer is option a.