Explanation:

- Option 1: A non-gratuitous promise

- Means a promise made with the expectation of receiving something in return.

- Not directly related to quantum meruit.

- Option 2: As gratuitous promise

- Means a promise made without expecting anything in return.

- Does not relate to quantum meruit, which involves an expectation of payment.

- Option 3: As much as is earned

- Refers to payment for services in an amount corresponding to what was actually done.

- Quantum meruit ensures fair compensation for work completed.

- Correct Answer

- Option 4: As much as is paid

- Implies getting an amount based on pre-defined payment rather than actual work done.

- Not aligned with the concept of quantum meruit, which is about compensating based on effort.