Step 1: Mangifera sylvatica is known mainly for dwarfing genes, not freestone character.
Step 2: Mangifera similis and Mangifera caesia are valued for other traits such as flavour, not specifically for stone freeness.
Step 3: Mangifera nicobarica is the wild species specifically linked to genes for easy flesh-seed separation.
Step 4: The correct answer is Mangifera nicobarica.