The correct answer is option 'B', which is 20 teeth.

Explanation:
The human mouth contains two sets of teeth during an individual's lifespan: the primary (baby) teeth and the permanent (adult) teeth.

1. Primary Teeth:
- The primary dentition consists of 20 teeth, which start to appear around 6 months of age.
- These teeth include 8 incisors (4 upper and 4 lower), 4 canines (2 upper and 2 lower), and 8 molars (4 upper and 4 lower).
- The primary teeth begin to fall out around the age of 6 or 7, making way for the permanent teeth.

2. Permanent Teeth:
- The permanent dentition consists of 32 teeth, which start to appear after the primary teeth fall out.
- These teeth include 8 incisors (4 upper and 4 lower), 4 canines (2 upper and 2 lower), 8 premolars (4 upper and 4 lower), and 12 molars (6 upper and 6 lower).
- The permanent teeth come in gradually, with the last set of molars, known as the third molars or wisdom teeth, typically appearing in the late teens or early twenties.

Therefore, if we subtract the number of primary teeth from the number of permanent teeth, we get the number of teeth that appear twice during an individual's lifespan.

32 (permanent teeth) - 20 (primary teeth) = 12 teeth

Out of the 32 permanent teeth, 20 are replacements for the primary teeth, while the remaining 12 teeth (third molars or wisdom teeth) appear later in life.

Hence, the correct answer is option 'B', 20 teeth.