Which of the following is a requirement for a valid will?

A valid will generally requires that it be signed by the testator, who is the person making the will, and typically also by witnesses. This requirement serves to establish the authenticity of the will and to ensure that the testator was of sound mind and not under duress when creating the document. The importance of the signatures lies in the formal acknowledgment of the testator's intentions regarding the distribution of their assets after death. Witnesses provide an additional layer of verification, confirming that the will was executed properly and that the testator was acting freely and voluntarily.

The other options, while they may seem relevant, do not capture essential legal requirements needed for a will to be considered valid. For instance, the requirement for a will to be written in a legal document can be subject to variations depending on state laws, as some jurisdictions may allow for oral wills under specific circumstances. Including a complete list of all assets is not necessary for validity, as a will can reference assets without exhaustive detail. Lastly, although a will may ultimately need to be filed with a court for probate purposes after the testator's death, this action is not a prerequisite for the validity of the will itself.