Which of the following is not a Lewis base?

Options

A. `CN^(Theta)`
B. `ROH`
C. `NH_(3)`
D. `AICI_(3)`

Explanation

To determine which of the following is not a Lewis base, we need to understand the definition of a Lewis base. A Lewis base is a species that can donate an electron pair to form a covalent bond.

Let's analyze the options provided:

1. **CN⁻ (Cyanide ion)**: This ion has a negative charge and possesses a lone pair of electrons. Therefore, it can easily donate this electron pair, making it a Lewis base.

2. **ROH (Alcohol)**: Alcohols have a lone pair of electrons on the oxygen atom. This lone pair can also be donated, which qualifies alcohols as Lewis bases.

3. **NH₃ (Ammonia)**: Ammonia has a lone pair of electrons on the nitrogen atom. It can donate this pair, thus acting as a Lewis base.

4. **AlCl₃ (Aluminum chloride)**: Aluminum chloride does not have a lone pair to donate. Instead, it has an empty p-orbital and can accept electron pairs, which classifies it as a Lewis acid.

Based on this analysis, the species that is not a Lewis base is **AlCl₃**.

### Final Answer:
The correct option is **AlCl₃**.