In$-$memory DB

We are designing an in$-$memory DB that has to store key$-$value pairs. It needs to support $2$ functions Add $($insert a new key$-$value pair in the DB$)$ and Get $($get the value given the key$).$ Which

structure can be used to implement these functions optimally?

Pick ONE option

A fixed size array

A singly linked list

A hash table