The correct option is
C And
Option C: 'And' is a conjunction that connects phrases, clauses or words. It means 'in addition to'. It is relevant to the given sentence, connecting the words 'waterfalls' and 'beaches'. Hence option C is correct.
Option A: 'In' means 'inclusion within a space'. 'Waterfalls' are not included within 'beaches'.
Option B: 'On' refers to the top or surface of something. 'Waterfalls' are not at the top of 'beaches'.
Option D: 'At' indicates 'a point or place occupied in space'. There is no such connection between 'waterfalls' and 'beaches', except for an addition.
These words do not relate to the given sentence in meaning.
Hence options A, B and D are incorrect.