What is the frequency of the clock that is being used as the clock source for the timer?

• A. some externally applied frequency f‚Äö√Ñ√∂‚àö√ë‚àö¬•
• B. controller‚Äö√Ñ√∂‚àö√ë‚àö¬•s crystal frequency f
• C. controller‚Äö√Ñ√∂‚àö√ë‚àö¬•s crystal frequency /12
• D. externally applied frequency/12