Answer: B. ow in knowing
Explanation:
In Phonetics, 'ow', 'oa', and 'oe' usually produce the same sound when pronounced which is the long o sound.
This is why the 'ow' in 'knowing' is pronounced in the same way as the 'oa' in 'approach'. The 'ow' in 'downstairs' is not similar to them due to it's position in the word having changed its sound.
