Small islands formed at the river mouth is called estuary.

Here are the answers to the questions:

1. Small islands formed at the river mouth is called estuary.

   • An estuary is a partially enclosed coastal body of water where freshwater from rivers mixes with saltwater from the ocean. Small islands formed at a river mouth are typically part of a delta, which is a landform created by the deposition of sediment.
   • The correct option is False.

2. The development of the undersea rift mountain range is the result of a convergent plate movement.

   • Undersea rift mountain ranges, like the Mid-Ocean Ridge, are formed at divergent plate boundaries, where tectonic plates pull apart, allowing magma to rise and create new crust. Convergent plate movement involves plates moving towards each other.
   • The correct option is False.

3. The Himalayas mountain range is the result of a plate subduction between a continental plate and an oceanic plate.

   • The Himalayas were formed by the collision of two continental plates (the Indian Plate and the Eurasian Plate). While oceanic crust was involved in earlier stages, the mountain range itself is primarily a result of continental-continental collision, not oceanic-continental subduction.
   • The correct option is False.

4. At the Atlantic Ocean, there is a subduction between the oceanic plates to create the Mid-Atlantic Ridge.

   • The Mid-Atlantic Ridge is a divergent plate boundary where new oceanic crust is formed as plates move apart. Subduction occurs at convergent plate boundaries where one plate slides beneath another.
   • The correct option is False.