Concept:Animal pairs must match the specific male and female names. 'Chicken' is the species name, not the female of 'rooster'.
Explanation:Option A: stallion (male horse) - mare (female horse) is correct.
Option B: cob (male swan) - pen (female swan) is correct.
Option C: rooster (male chicken) - chicken is mismatched. The female chicken is called 'hen', not 'chicken'.
Option D: billy (male goat) - nanny (female goat) is correct.
Only option C uses a general term instead of the specific female name. So it is the mismatched pair.
Answer:Option C – rooster - chicken