Answer:
The correct answer will be option-A
Step-by-step explanation:
A scientific experiment is designed in a way that it tests the predicted hypothesis and so contains the three different types of variables namely: independent (can be modified), dependent (to be studied) and the control (kept constant and unchanged).
In the given question, since the scientists studied the effect of nitrate and phosphate in which he provided the plant samples with nitrate and phosphate but he missed the control sample which should lack the variable to be tested that is nitrate and phosphate. This control variable helps the scientist to study the relationship between other samples.
Thus, option-A is the correct answer.