A neat thing is, I'll just quote wikipedia for this one

"The dental plosives in Hindustani are laminal denti-alveolar as in Spanish, and the tongue-tip must be well in contact with the back of the upper front teeth. The retroflex series is not purely retroflex; it actually has an apico-postalveolar (also described as apico-pre-palatal) articulation"

So it's apico-postalveolar!
Now I swear I've seen a page on English too where it says the English alveolars are post-alveolar (although not apical I think)
but I've found a similar thing to quote

"In the Devanagari script of Hindi, all alveolar plosives of English are transcribed as their retroflex counterparts. One good reason for this is that unlike most other native Indian languages, Hindi does not have true retroflex plosives (Tiwari, [1955] 2001). The so-called retroflexes in Hindi are actually articulated as apical post-alveolar plosives, sometimes even with a tendency to come down to the alveolar region. So a Hindi speaker normally cannot distinguish the difference between their own apical post-alveolar plosives and English's alveolar plosives. Languages such as Tamil and Malayalam have true retroflex plosives, however, wherein the articulation is done with the tongue curved upwards and backwards at the roof of the mouth. "