A will must typically be:

A valid will must be signed by the testator, which is the person making the will, and it typically also needs to be witnessed to ensure its authenticity and to provide legal backing in case of disputes. Witnesses confirm that the testator was of sound mind and that the signing was done voluntarily and without coercion. Requirements for witnessing can vary by jurisdiction, but generally, having this signature and witness process is essential to prevent fraud and to uphold the legal integrity of the document.

The other options touch on aspects that may be relevant in certain contexts but are not universally required for a will to be legally valid. For example, while a will could be completed in one sitting, this is not a legal requirement, as individuals may take time to draft and revise their wishes. Specific formatting is not usually mandated, as the critical elements are the testator’s signature and witnessing. Lastly, reviewing a will by a tax advisor is beneficial for tax implications but is not a necessity for the will's validity itself.