1. Recall the definition of a racemic mixture

A racemic mixture contains equal amounts of two enantiomers of a chiral compound.

If the two enantiomers are represented as $R$ and $S$, then a racemic mixture is:

$\text{Racemic mixture} = 50\%\,R + 50\%\,S$

Such a mixture is optically inactive because the rotation caused by one enantiomer is cancelled by the other.

2. Understand the terms in the options

• Isomeric compounds: very broad term; includes many kinds of isomers, not specifically the pair needed for a racemic mixture.
• Enantiomers are a pair of optical isomers that are non-superimposable mirror images.
• Meso compounds are internally compensated and do not form racemic mixtures by mixing two different species.

3. Evaluate each option

• A: isomeric compounds
  Incorrect. A racemic mixture is not formed by just any isomeric compounds.

• B: enantiomers with chiral compounds
  This wording is not standard. A racemic mixture is formed by mixing two enantiomers of a chiral compound in equal amounts. If interpreted literally, this option is awkward/incomplete.

• C: meso compounds
  Incorrect. A meso compound is itself achiral and does not require mixing two species to give optical inactivity.

• D: optical isomers
  Correct. A racemic mixture is formed by mixing two optical isomers (i.e., enantiomers) in equal amounts.

4. Conclusion

The correct choice is:

$\boxed{D}$

because a racemic mixture consists of two optical isomers (enantiomers) present in equal proportion.