Final answer:
The falx cerebri attaches to the crista galli, a component of the ethmoid bone, making option (a) Crista galli the correct answer.
Step-by-step explanation:
The attachment site for the falx cerebri is the crista galli, which is a small upward projection located at the midline in the floor of the anterior cranial fossa; it is formed by the ethmoid bone. The foramen magnum is a large opening in the occipital bone of the skull through which the spinal cord emerges, and it is not the attachment site for the flax cerebri. Therefore, the correct answer is (a) Crista galli.