Step 1: Understanding the Concept:
This question falls under "Error Detection" and specifically targets the "Stative vs. Dynamic Verbs" rule in English grammar.
Most verbs represent actions (e.g., run, eat, speak) and can be used in the continuous/progressive tense (e.g., I am running). These are called dynamic verbs.
However, some verbs represent a state of being, a mental process, or a feeling (e.g., know, believe, like, hate, understand). These are called "Stative Verbs."
Step 2: Detailed Explanation:
The fundamental rule for stative verbs is that they are almost never used in the continuous form (verb + ing) in standard formal English.
1. Analysis of 'Know':
"To know" is a mental state. You either have the information in your head, or you do not.
Knowledge is not a physical action that you can be "in the middle of doing."
Therefore, saying "I am knowing" is grammatically incorrect. One should say "I know."
2. Examining the Sentence Parts:
(A) 'is knowing': This is a present continuous construction of a stative verb. This is the primary error. It should be changed to the simple present "He knows" or the simple past "He knew."
(B) 'answer': This is a noun acting as the object of the verb. It is used correctly.
(C) 'refused': This is the simple past tense of "refuse." It indicates a past action. While mixing "is knowing" (present) and "refused" (past) also creates a tense consistency issue, the use of "is knowing" is a more fundamental violation of verb category rules.
(D) 'tell': This is the base form used after the infinitive marker "to" (to tell). This is grammatically sound.
3. Correcting the Sentence:
The corrected sentence should read: "He knew the answer but refused to tell us."
(This makes the sentence consistent in the past tense).
Alternatively, if the state is current: "He knows the answer but refuses to tell us."
Step 3: Final Answer:
The error lies in the phrase "is knowing" because the verb 'know' cannot be used in a continuous tense. Hence, Option (A) is the correct answer.