Final answer:
An Affidavit of Execution is the document that can prevent problems verifying the validity of a will when it is signed, as it serves as sworn testimony by the witnesses of the will's authenticity.
Step-by-step explanation:
The document that can be prepared at the time of signing a will that will help prevent later problems in verifying the validity of the will is an Affidavit of Execution. This legal document is completed by the witnesses to the will, stating that they were present and saw the will-maker (testator) sign the will and that the will-maker appeared to be of sound mind and free from duress. Having an Affidavit of Execution can be beneficial as it can greatly simplify the probate process since it acts as sworn testimony of the will's authenticity, thus potentially reducing the risk of legal challenges.
Answer: D. Affidavit of Execution.