Final answer:
The correct answer is b. Assignment. The allowable aggregate operation on a struct is assignment. Structs can have their values copied over from one to another if they are of the same type, while other operations must be defined by the user.
Step-by-step explanation:
The question asked is which of the following is an allowable aggregate operation on a struct: a. Arithmetic, b. Assignment, c. Input/output, d. Comparison. The correct answer is b. Assignment. Structs in programming languages like C and C++ support assignment operations, meaning you can assign the values of one struct to another provided they are of the same type. However, aggregate operations like arithmetic and comparison must be defined explicitly with operator overloading or by comparing each member individually. Input/output operations are not directly supported as aggregate operations but can be implemented by the user for their specific needs.
Assignment can also be done with structs, where one struct can be assigned to another struct of the same type. Input and output operations like reading from or writing to a file or the console can be performed on struct objects. Finally, comparison operations like checking if two structs are equal or not can be done using operators like == and !=.