Which document is essential for proving the valid execution of a will during probate?

The signed affidavit by the witnesses is essential for proving the valid execution of a will during probate because it serves as a formal statement confirming that the will was executed in accordance with the legal requirements. Witnesses play a crucial role in the probate process, as they can testify about the circumstances under which the will was signed, affirming that the testator was of sound mind, acting voluntarily, and that the will was signed in their presence. An affidavit can provide a clear and documented account of these factors, which is often required by the court to establish the will's validity.

The other options do not fulfill this purpose. A testator's birth certificate, while useful for establishing identity and age, does not demonstrate the execution of the will. A letter from the estate's accountant may provide financial insights but does not pertain to the will's execution. Lastly, the last tax return of the testator is not relevant to the will's validity; it may provide information on the testator's financial situation but does not verify whether the will was properly executed.