They represent two different conformations (conformers) of the same molecule, not different stereoisomers.​
So, the correct option is d. conformer.

Understanding the Newman projections

Both drawings are Newman projections of a single C–C bond in the same compound, showing CH₃, Cl, Br, OH, and H attached around that bond. Free rotation about a single sigma bond allows these groups to adopt various staggered arrangements without breaking any bonds, so such structures are called conformations and each distinct arrangement is a conformer.​

Why they are conformers

In each projection, the front and back carbons have exactly the same substituents and connectivity; only the dihedral angles between groups differ by rotation around the C–C bond. Because simple bond rotation interconverts the two, they are not different molecules but different shapes (conformations) of the same molecule, hence conformers.​

Why not diastereomers

Diastereomers are stereoisomers that are not mirror images and cannot be interconverted by simple bond rotation; at least one but not all stereocenters have opposite configuration. Here, no stereocenter’s configuration is changed; only the conformation about a single bond changes, so the pair does not qualify as diastereomers (option a).​

Why not enantiomers

Enantiomers are non‑superimposable mirror images with all stereocenters inverted. The two Newman projections are not mirror images; instead, rotating one around the C–C bond makes it identical to the other, so they are not enantiomers (option b).​

Why not tautomers

Tautomers are structural isomers that differ by movement of a proton and a shift of a multiple bond (for example, keto–enol pairs). In these projections no proton transfer or double‑bond rearrangement occurs, so option c (tautomer) is also incorrect.​

SEO‑friendly introduction

Understanding the relationship between a pair of chemical entities drawn as Newman projections is essential for mastering stereochemistry and conformational analysis. By analysing substituents, connectivity, and possible bond rotations, it becomes straightforward to decide whether two structures are diastereomers, enantiomers, tautomers, or simply conformers of the same molecule.​