None, there's no American alternative. Not because they don't measure up, US missiles, especially in missile-defense, are the best around, but because the American's don't produce a land-based missile like S-400 due to a lack of necessity and differing doctrine/tactics.
Their only export land-based SAM system is PAC-2. It's not in the same class as S-400. It's range is far shorter.
Apart from that they offer several land-based anti-missile systems including PAC-3, which is a low-end missile defense weapon, for terminal defense against short and tactical ranged ballistic missiles.
And THAAD, which is among the best, if not the best land-based counter missile missile, with counter MRBM and IRBM capabilities
Other land-based options like stinger are either far from being in the class of S-400, or not exportable, as in the case of AEGIS Ashore, which is also for counter missile duties, not general air-defence.
So far only the US and Japan have been allowed AEGIS Ashore.
Which makes use of a land-based SM-3 Block IIA.
Operational range Block IIA ~2500 km (1350 nautical miles)
Flight ceiling Block IIA ~1500 km (933 miles)
Speed Block IIA ~4.5 km/s (Mach 15.25)
US sea based missiles were never an option for India, not that even India desired them at this point. It's current batch of Barak 1 and Barak 8 are not substitutes for SM-2, SM-3 or SM-6. Barak 1 and Barak 8 perform less effectively then the Standard series with significantly less range. They aren't bad missiles, they are very good, but aren't in the class of the Standard series.
While BDForever may dismay at the S-400, which is damn capable, there was no American alternative that could be offered to India. SM-6 is the closest, and better performing, alternative, but it's sea based. SM-3 outclasses all competitors in counter-missile operations.