Final answer:
Alkaline hydrolysis with sodium hydroxide (NaOH) is the best option to hydrolyse a phosphonate diester to free phosphonic acid under relatively mild conditions, in contrast to the harsher acid hydrolysis using concentrated sulfuric acid.
Step-by-step explanation:
To hydrolyse a phosphonate diester to free phosphonic acid under relatively mild conditions, one can utilize alkaline hydrolysis with sodium hydroxide (NaOH). Alkaline hydrolysis is favored in this context as it is typically gentler and more selective compared to the harsh conditions of acid hydrolysis with concentrated sulfuric acid.
On the contrary, acid hydrolysis, while capable of hydrolyzing esters, often requires high temperatures and strong acids, which may not be considered mild conditions. Acidic hydrolysis is a reversible reaction and may not proceed to completion without rigorous conditions.