Complete the following idiom by choosing the correct word: "Let the cat out of the ______."

Introduction / Context:
This question tests knowledge of a very common English idiom: let the cat out of the bag. Idioms are fixed expressions whose words usually cannot be changed without losing the established meaning. Here, you must know which noun completes the expression so that it retains its accepted figurative sense, which is to reveal a secret.

Given Data / Assumptions:

- Incomplete idiom: Let the cat out of the ______. - Options: house, well, bag, cart. - Only one noun forms a standard, widely recognised idiom in English.

Concept / Approach:
The complete idiom is let the cat out of the bag, which means to reveal a secret, often by mistake. It does not literally refer to cats or bags in most contexts, but rather to information being released. Therefore, the correct word to fill the blank is bag. Tried with house, well, or cart, the phrase loses its idiomatic identity and becomes either nonsensical or merely literal, which is not what idiom questions are testing.

Step-by-Step Solution:
Step 1: Recall the fixed phrase from everyday usage: Someone let the cat out of the bag about the surprise party. Step 2: Compare each option. Let the cat out of the bag matches the remembered expression exactly. Step 3: Try house: Let the cat out of the house simply describes a physical action, not an idiom about secrets. Step 4: Try well: Let the cat out of the well sounds odd and has no recognised figurative meaning. Step 5: Try cart: Let the cat out of the cart is also not an established phrase in English. Step 6: Conclude that bag is the only option that completes the idiom correctly.

Verification / Alternative check:
Consider how people explain the idiom in dictionaries or language guides. It is always presented as let the cat out of the bag, meaning reveal a secret, usually accidentally. This consistent form confirms that bag is the required noun. In exam situations, you can also recall whether you have seen the complete phrase previously in books, subtitles, or conversations; if so, that remembered version is almost always the correct one.

Why Other Options Are Wrong:
House, well, and cart are perfectly valid nouns, but they do not combine with let the cat out of the in any idiomatic or widely accepted way. Using them changes the phrase into an ordinary literal description instead of an idiom about secrets, and such changes defeat the purpose of the question, which is to test knowledge of fixed expressions.

Common Pitfalls:
Some learners overthink idiom completion questions and try to create novel images, imagining that a cat might be stuck in a well or a cart. However, idioms are conventional, not creative, in exam contexts. The safest strategy is to memorise common idioms and recall them exactly, without trying to invent new versions. This prevents you from being distracted by options that seem imaginative but are not standard English.

Final Answer:
bag is the correct word, giving the complete idiom let the cat out of the bag.