Among `PCI_(3),CH_(3)^(+),NH_(2)^(-)` and `NF_(3)` is least reactive towards water.

Explanation

To determine which of the given species (PCl3, CH3^+, NH2^-, and NF3) is least reactive towards water, we need to analyze their polarity and how that affects their reactivity with water, which is a polar solvent.

### Step-by-Step Solution:

1. **Understanding Polarity**:
- Water is a polar solvent due to the significant electronegativity difference between oxygen and hydrogen, leading to a partial negative charge on the oxygen and partial positive charges on the hydrogens.

2. **Analyzing Each Species**:
- **CH3^+ (Methyl Cation)**: This species has a positive charge, making it highly polar. It will readily react with water due to its strong interaction with the polar solvent.
- **NH2^- (Amide Ion)**: This species has a negative charge, which also makes it highly polar. Similar to CH3^+, it will react quickly with water.
- **PCl3 (Phosphorus Trichloride)**: PCl3 is a polar molecule due to the difference in electronegativity between phosphorus and chlorine. It will react with water, but not as readily as the charged species.
- **NF3 (Nitrogen Trifluoride)**: Although NF3 is polar, the electronegativity difference between nitrogen and fluorine is not as substantial as that between phosphorus and chlorine. Therefore, NF3 is less polar and less reactive towards water.

3. **Comparing Reactivity**:
- The charged species (CH3^+ and NH2^-) are the most reactive due to their strong polarity.
- PCl3 is more reactive than NF3 because it is more polar.
- NF3, being the least polar, is the least reactive towards water.

4. **Conclusion**:
- Among the four species, **NF3** is the least reactive towards water due to its lower polarity compared to the others.

### Final Answer:
**NF3 is the least reactive towards water.**