Which one of the following is not a concept of Lamarck?

The correct answer is Option (3) → Rate of survival of organisms is different due to variations.

Lamarck said evolution was driven by the use and disuse of organs: an organ used again and again grows larger, and the change gained in a lifetime is passed to the young. Different survival of organisms because of variations is not his idea; it belongs to Darwin’s theory of natural selection.

Option (1) If an organ is used continuously it will increase in size. — Incorrect. This is Lamarck’s idea of use and disuse. In his giraffe example, the neck grew longer from stretching to reach leaves on tall trees.

Option (2) Inheritance of acquired characters. — Incorrect. This is also Lamarck’s: the giraffes passed on the acquired character of an elongated neck to their young.

Option (3) Rate of survival of organisms is different due to variations. — Correct. This is a Darwinian idea. Members of a population vary, those with useful heritable variations survive and leave more progeny, and so they are selected by nature.

NCERT Extract:
Biological Evolution – “… Even before Darwin, a French naturalist Lamarck had said that evolution of life forms had occurred but driven by use and disuse of organs. He gave the examples of Giraffes who in an attempt to forage leaves on tall trees had to adapt by elongation of their necks. As they passed on this acquired character of elongated neck to succeeding generations, Giraffes, slowly, over the years, came to acquire long necks. Nobody believes this conjecture any more.”
Evolution of Life Forms – A Theory – “… Those characteristics which enable some to survive better in natural conditions (climate, food, physical factors, etc.) would outbreed others that are less-endowed to survive under such natural conditions.”