In the conversion of ethanol to ethene, the first step of the reaction involves the formation of

Oxonium ion

The correct answer is Option (1) → Oxonium ion

Ethanol has a poor leaving group ($-OH$). Acid converts it into a better leaving group (water).

Step 1 (First step): Protonation

$CH_3CH_2OH+ H^+ → CH_3CH_2OH_2^+$

This protonated alcohol is called an oxonium ion.

After this:

• Water leaves
• Carbocation forms
• Then elimination gives ethene

Explanation of each option

1. Oxonium ion

Formed when ethanol's oxygen gets protonated in acidic medium. This converts $-OH$ into $-OH_2^+$,making water a good leaving group. This is always the first step in acid-catalysed dehydration of alcohols.

2. Carbonium ion

Carbocation forms only after water leaves from the oxonium ion. So this is a later intermediate, not the first one.

3. Carbene

Carbenes are neutral divalent carbon species (like: $CH_2$). They are not formed in alcohol dehydration reactions.

4. Carbanion ion

Carbanions are negatively charged carbon species, formed in strongly basic conditions. This reaction occurs in acidic medium, so impossible.