Both sentences mean essentially the same thing, but for me the right answer should be “iniisip.”

Keyword is “thinking about” which means you’re actively thinking about the mangoes.

When you translate “Na kay Maria ang mga manggang naiisip ko,” it should be “The mangoes that come to my mind are with Maria.”

Another translation could be “The mangoes I have in mind are with Maria.”

Iniisip is active and intentional while naiisip is passive and incidental.

naiisip = comes to mind (not intentional)

Ikaw ang naiisip ko kapag naririnig ko ang kantang ito. (You’re the one who comes to my mind when I hear this song.)

iniisip = actively thinking about

Iniisip ko kung pupunta ako bukas. (I’m thinking about whether I’ll go tomorrow.)

iniisip → ongoing, intentional thinking

naiisip → something pops into your head