Question Analysis: This question pertains to the Displacement Method experiment (Bessel's method) used to determine the focal length of a convex lens, where two positions of the lens produce a sharp image on a fixed screen.

Step 1 (Governing Principle): [0.25 Mark]
According to the principle of reversibility of light in the displacement method, for a fixed distance $D$ between the object and the screen, if $D > 4f$, there exist two positions of the convex lens at distances $u_1$ and $u_2$ from the object such that: $$u_1 = v_2 \quad \text{and} \quad u_2 = v_1$$ where $u$ is the object distance and $v$ is the image distance.

Step 2 (Mathematical Working): [0.5 Mark]
The magnification $m$ is given by the ratio of image size to object size: $$m = \frac{v}{u}$$ For the first position (lens closer to the object): $u_1 < f$, resulting in $v_1 > u_1$, hence $m_1 = \frac{v_1}{u_1} > 1$ (Enlarged).
For the second position (lens closer to the screen): $u_2 > f$, resulting in $v_2 < u_2$, hence $m_2 = \frac{v_2}{u_2} < 1$ (Reduced).
Since the question asks for the two positions respectively, the sequence is enlarged, reduced.

Step 3 (Final Concluded Answer): [0.25 Mark]
The correct option is (D) enlarged, reduced.

💡 EL10 Concept Insight: Think of the lens like a seesaw; when the lens is closer to the object, it "stretches" the image to make it bigger, but when you slide the lens closer to the screen, it "squeezes" the light rays together to make the image smaller.