1. The question asks: "Following types of compounds (as $I, II$) are studied in terms of isomerism in :" and the options are:

   • A: chain isomerism
   • B: position isomerism
   • C: conformers
   • D: stereoisomerism

2. Although the actual structures labeled $I$ and $II$ are not visible in the prompt, this standard question format in basic organic chemistry usually refers to two compounds having:

   • same molecular formula,
   • same connectivity,
   • but different spatial arrangement.

3. Such compounds are not classified under:

   • chain isomerism: different carbon skeletons,
   • position isomerism: same skeleton but different position of functional group/multiple bond,
   • conformers: rotamers obtained by rotation about single bond.

4. If the difference is due to spatial arrangement and the compounds are studied under isomerism, then the correct category is:

$\text{stereoisomerism}$

5. Hence, the correct option is:

$\boxed{D}$