Analyzing the pairs of compounds in List-I:
A. 2-Methylpropene (CH3−C(CH3)=CH2) and but-1-ene (CH3−CH2−CH=CH2) have the same molecular formula C4H8 but different carbon skeletons (branched vs straight chain). Thus, they are chain isomers. (A-III)
B. Cis-but-2-ene and trans-but-2-ene are geometrical isomers, which fall under the category of stereoisomers. (B-I)
C. 2-Butanol (C4H9OH) and diethyl ether (C2H5−O−C2H5) have the same molecular formula C4H10O but different functional groups (alcohol vs ether). Thus, they are functional group isomers. (C-IV)
D. But-1-ene and but-2-ene have the same carbon chain and functional group but differ in the position of the double bond (at C1 vs C2). Thus, they are position isomers. (D-II)
Matching the pairs: A-III, B-I, C-IV, D-II.