If in a garden, there are 20 sunflowers plants of last year and through reproduction 8 new plants are added. Then, the birth rate is :

0.4 offspring per sunflowers per year

The correct answer is Option (3) - 0.4 offspring per sunflowers per year

In the given scenario, there are 20 sunflowers plants from the previous year, and through reproduction, 8 new plants are added. The birth rate is calculated by dividing the number of new plants by the initial number of sunflowers plants.

In this case, the birth rate is 8 new plants divided by 20 existing plants. Simplifying the calculation, we find that the birth rate is 0.4 offspring per sunflowers plant per year.

Therefore, the correct answer is (3) 0.4 offspring per sunflowers per year, as it accurately represents the birth rate in this specific situation.