Ester is formed by the reaction between:

Options

A. An acid and an alcohol
B. An acid and a base
C. A base and an alcohol
D. An acid and an alkene

Explanation

To determine what an ester is formed from, we need to understand the reaction involved in its formation. Here’s a step-by-step breakdown of the process:

### Step 1: Identify the Components
Ester formation involves two main components:
- An **alcohol** (R-OH)
- A **carboxylic acid** (R'-COOH)

### Step 2: Understand the Reaction
The reaction between an alcohol and a carboxylic acid is known as **esterification**. During this reaction, the hydroxyl group (-OH) from the acid and a hydrogen atom from the alcohol combine to form water (H2O), and the remaining parts of the alcohol and acid form the ester.

### Step 3: Write the General Reaction
The general reaction can be represented as:
\[ \text{R-OH (alcohol) + R'-COOH (acid)} \rightarrow \text{R'-COOR (ester) + H2O} \]

### Step 4: Example of Ester Formation
For instance, if we take ethanol (CH3CH2OH) as the alcohol and ethanoic acid (CH3COOH) as the acid, the reaction would be:
\[ \text{CH3CH2OH + CH3COOH} \rightarrow \text{CH3COOCH2CH3 (ethyl acetate) + H2O} \]

### Conclusion
Thus, we can conclude that an ester is formed by the reaction between an **alcohol** and a **carboxylic acid**.