Explanation:

- The mob:

- No issues here. "The mob" is a correct noun phrase referring to a group of people.

- is believing:

- This segment contains the error.

- "Is believing" suggests a continuous tense, which is inappropriate for the verb "believe," a stative verb that does not traditionally take continuous tenses.

- Correct usage would be "believes."

- This is the correct answer.

- Every word:

- Grammatically correct. It is a noun phrase functioning as the object of what is believed.

- The leader says:

- Correct as well. This phrase acts as a relative clause, modifying "every word."