Formal:
* Credible: This is a common and neutral word that simply means believable.
* Plausible: This word suggests that something is likely or possible, making it believable.
* Authentic: This word emphasizes the genuineness and truthfulness of something, making it believable.
* Verifiable: This word means that something can be proven to be true, making it believable.
* Trustworthy: This word implies that someone or something is reliable and can be trusted to be truthful, making it believable.
Informal:
* Believable: This is a straightforward and simple word meaning able to be believed.
* Convincing: This word suggests that something is persuasive and likely to be believed.
* Sound: This word can be used to describe a logical and believable argument.
* Valid: This word implies that something is true and justifiable, making it believable.
The best word to use will depend on the specific context and the desired tone.