Mendelian theory of inheritance was crucial for Darwin-Wallace's theory of natural selection because

• Correct answer
  A

  it explained how new variations will persist in a population over time despite their low frequency, which is essential for natural selection.
• B

  the Mendelian theory explained how characters acquired over an organism's lifetime can be passed on to the next generation, which is essential for natural selection to operate.
• C

  the Mendelian theory explained how variation appears in the biological system which is crucial for evolution by the process of natural selection.
• D

  the Mendelian theory first explained that linkage is essential for natural selection.

Hence, the correct answer is option: 'A'.