K Missile family

The K family of missiles, named after Indian scientist and former president A. P. J. Abdul Kalam, is a family of submarine-launched ballistic missiles (SLBM) developed by India to boost its second strike capabilities and thus augment its nuclear deterrence. Information about this family of missiles has mostly been kept classified. It is reported that "K" missiles are faster, lighter and stealthier than their Agni missile counterparts.
Missiles in the series.
K-15 or Sagarika missile.
The "Sagarika"/K-15 missile (Sanskrit: सागरिका, IAST:Sāgarikā, meaning "Oceanic") is the SLBM version of the land-based Shaurya missile. With a shorter range than K-4 missiles it is to be integrated with Arihant class submarine concurrently developed for the use of Indian Navy.
Sagarika/K-15 was developed at the DRDO’s missile complex in Hyderabad. The complex consists of the Defence Research and Development Laboratory (DRDL), the Advanced Systems Laboratory (ASL) and the Research Centre, Imarat (RCI). <br>DRDL designed and developed the missile, while the ASL provided the motors and propulsion systems. The RCI’s contribution was in avionics, including control and guidance systems and inertial navigation systems.
Medium range K-15 ballistic missile has a range between 700  km to 1,500 km with varying payload. This will also get help from Indian Regional Navigation Satellite System (IRNSS) to ensure guaranteed national access to precision navigation. These will enable high accuracy required for precision strike. The last developmental test of the missile was conducted on 28 January 2013 from an under water launch platform off the coast of Visakhapatnam .
K-4 Missile.
K-4 is an intermediate-range submarine-launched ballistic missile under development by DRDO. It is a 10 m long missile weighing 20 tonnes, capable of carrying a 2 tonne payload up to a range of 3,500 km. INS Arihant, first of the Arihant Class Submarines, will be able to carry 4 K-4 missiles. The K-4 missile was successfully tested on 24 March 2014 from an underwater pontoon submerged 30 m deep. India successfully test fired the 3,500 km strike range nuclear-capable K-4 submarine-launched ballistic missile off the coast of Andhra Pradesh on 19 January 2020.
K-5 Missile.
K-5 missile is reportedly being developed by Defence Research and Development Organisation (DRDO) for the Indian strategic forces’ underwater platforms. It will arm the future variants of Arihant class submarines of the Indian Navy. Reportedly, DRDO is in the process of developing a submarine-launched solid fuel missile with a maximum range of 5,000 kilometres. In October 2020, "Hindustan Times" reported that the missile is currently under development and is expected to be tested by 2022
K-6 Missile.
K-6 missile is SLBM which is reportedly under development by Defence Research and Development Organisation (DRDO)'s Advanced Naval Systems Laboratory in Hyderabad. It is a three-stage solid fuel MIRV capable missile with a length of 12m, width of 2m, a payload of 2-3 tonne warhead and a range of around 6,000 to 8000 km. It will arm the of ballistic missile submarines of the Indian Navy.
Significance.
These 'K' missiles are intrinsically important for India's nuclear deterrence arsenal because they provide India with a much needed ideal and invulnerable second-strike capability stated in India's Nuclear Doctrine and thus shift the balance of power in India's favour in Asian region.