Frame Challenge: Take the Dodge action.

Taking the Dodge action achieves the desired result, and more:

When you take the Dodge action, you focus entirely on avoiding attacks. Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage.

This is way better than using your action to Ready dropping Prone:

• A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
• The creature has disadvantage on attack rolls.
• An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.

Just take the Dodge action.

Taking the Dodge action achieves the desired result, and more:

When you take the Dodge action, you focus entirely on avoiding attacks. Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage.

This is way better than using your action to Ready dropping Prone:

• A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
• The creature has disadvantage on attack rolls.
• An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.

This has the further advantage of not having to rely on your DM to approve the wording of the trigger for your readied action or its timing.

Frame Challenge: Take the Dodge action.

Taking the Dodge action achieves the desired result, and more:

When you take the Dodge action, you focus entirely on avoiding attacks. Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage.

This is way better than dropping Prone:

• A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
• The creature has disadvantage on attack rolls.
• An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.

Frame Challenge: Take the Dodge action.

Taking the Dodge action achieves the desired result, and more:

When you take the Dodge action, you focus entirely on avoiding attacks. Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage.

This is way better than using your action to Ready dropping Prone:

• A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
• The creature has disadvantage on attack rolls.
• An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.