Final answer:
Field curing is the curing of concrete test specimens in the same conditions as the structure, commonly for accurate compressive strength testing in controlled conditions.
Step-by-step explanation:
Field curing is the method of curing concrete test specimens in the same environment as the concrete in the structure so they undergo similar conditions. This method is particularly used when test specimens are to be employed for compressive strength testing in controlled conditions. When the specimens are cured in this way, they better represent the in-situ concrete's character for the intended evaluation of compressive strength, facilitating a more accurate prediction of how the concrete will perform in the actual structure.
For this reason, the answer to the student's question is that field curing is used when test specimens are to be used for d) compressive strength testing in controlled conditions.