Ram Nath Kovind became the first Indian President to set foot in Croatia when he travelled there in March 2019 as part of a wider tour of Bolivia, Chile and Croatia, a trip meant to widen India's economic and cultural links with these countries.
Ram Nath Kovind therefore holds the distinction of being the first Indian head of state to visit Croatia.
This visit is easiest to place by recalling it as part of a specific three-nation tour rather than a standalone trip, then checking which President that tour belonged to.
Because this particular multi-country tour is tied specifically to Kovind's presidency, he is confirmed as the first Indian President to visit Croatia.
So the correct answer is Ram Nath Kovind.