Final answer:
To facilitate the trimming of brittle toenails, soaking feet in warm water is the best action as it softens the nails, making them easier to cut without breaking.
Step-by-step explanation:
The action that will facilitate the trimming of brittle toenails is a) Soaking feet in warm water. This process softens the nails, making them easier to trim without breaking. It is important to be gentle and use proper nail-cutting tools to prevent any damage to the nails or skin. Applying oil may help condition the nails but soaking is the direct method that facilitates easier trimming. A pumice stone is generally used for removing dead skin and calluses, and is not used for trimming nails. Therefore, the correct response is a).