(B) Both plants are covered with separate wooden bell-jars.
The incorrect step is option (B). The setup uses glass bell-jars, not wooden ones. Wooden bell-jars would block sunlight, preventing photosynthesis, making the experiment invalid.
This question is from the photosynthesis experiment (testing for starch). The bell-jars must be transparent (glass) so that sunlight can reach the plants. One jar contains KOH to absorb CO₂ (plant I — no photosynthesis), while plant II serves as control. All other options (A, C, D) describe correct steps. Examiners expect students to identify "wooden" as the error, since wood is opaque.