The question asks about the number of optical isomers possible for 2, 3-pentanediol. To understand this, let's first define what optical isomers are.

Optical isomers, also known as enantiomers, are stereoisomers that are non-superimposable mirror images of each other. In other words, they have the same molecular formula and connectivity but differ in their spatial arrangement. These isomers usually occur in compounds with one or more chiral centers, which are carbon atoms that have four different substituents attached to them.

Now, let's analyze the structure of 2, 3-pentanediol to determine the number of chiral centers and, therefore, the number of possible optical isomers.

Structure of 2, 3-pentanediol:
```
H H H H OH
| | | | |
H - C C C C C - OH
| | | | |
H H H OH H
```

From the structure above, we can see that there are two carbon atoms (C-2 and C-3) that are chiral centers. Each of these carbon atoms is bonded to four different substituents (H, OH, H, and another carbon atom). Therefore, each chiral center can have two possible configurations, resulting in a total of 2 x 2 = 4 possible optical isomers.

Therefore, the correct answer is option 'C' (4). There are four possible optical isomers for 2, 3-pentanediol.