Step 1: Restate the problem as a dilution.
We start with 2 kg of a herbicide product that is 50% pure atrazine by weight, and we stir this into 100 litres of water to make a spray mixture. We want to know how much of the final mixture, by weight, is actual atrazine.
Step 2: Work out how much actual atrazine is in the 2 kg of product.
Since the product is labeled 50% a.i., half of every kilogram is atrazine and half is inert filler.
$ 2 \text{ kg} \times 0.50 = 1 \text{ kg of atrazine} $
Step 3: Set the total mixture on the same weight scale.
1 litre of water weighs close to 1 kg, so 100 litres of water weighs close to 100 kg. The extra 2 kg of formulation stirred in is small next to 100 kg, and for this kind of concentration sum it is standard practice to take the total mixture weight as the water weight, 100 kg.
Step 4: Divide the atrazine weight by the total mixture weight.
$ \text{Concentration} = \frac{1 \text{ kg}}{100 \text{ kg}} \times 100 = 1\% $
Step 5: Cross check against the other answer choices.
If the water volume had instead been 1000 litres, the same 1 kg of atrazine would spread across 1000 kg of water, giving 0.1%. If it had been 500 litres, it would give 0.2%. Only the 100 litre case lines up cleanly with one of the four choices, which is 1.0%.
Step 6: Final Answer.
The spray solution contains atrazine at a concentration of 1.0%.
\[ \boxed{1.0\%} \]