Step 1: Use the combined-power rule.
Two thin lenses in contact form an equivalent lens whose power is the algebraic sum of the individual powers, \(P = P_1 + P_2\), with power \(P = 1/f\) measured in dioptres.
Step 2: Write the two powers with a common magnitude.
Both lenses have equal focal-length magnitude \(F\). Convex gives \(P_1 = +1/F\); concave gives \(P_2 = -1/F\). They are equal in size but opposite in sign.
Step 3: Cancel.
Adding, \(P = 1/F - 1/F = 0\). The converging action of the convex lens is exactly undone by the diverging action of the concave lens.
Step 4: Physical meaning.
Zero power corresponds to infinite focal length: rays pass through without net bending, just as through a flat sheet of glass. The combination therefore acts as a plane plate, option (iv).
\[\boxed{F_{comb} = \infty,\ \text{acts as plane plate}}\]