Final answer:
A word-reading survey would provide the most useful information on how to help a student with phonics and decoding skills.
Step-by-step explanation:
To assess a student's phonics and decoding skills, the most useful informal diagnostic assessment would be a word-reading survey to show which sound-symbol correspondences the student knows and which ones still need practice. This type of assessment would provide valuable information on the specific sound-symbol correspondences that the student has mastered and those that require further practice. It would help identify the areas where the student needs additional support and interventions to improve their phonics and decoding skills.