The problem asks to choose the appropriate number to complete the missing part of the third circle. Let's examine the relationships between the numbers in the circles.

First Circle:

• Numbers: 3, 7, 4
• Observing the relationship:
  • $3 \times 4 = 12$, then $12 + 7 = 19$
  • There's no immediate obvious pattern here.

Second Circle:

• Numbers: 6, 21, 16
• Possible relationship:
  • $6 \times 16 = 96$, then $96 - 21 = 75$
  • No obvious match again.

This doesn't seem productive, so let's check another possible relationship:

In each circle:

1. The top number is the difference between the sum of the bottom numbers.

For the first circle:

• $7 + 4 = 11$, and $11 - 3 = 8$ doesn't fit a clear relationship.

For the second:

• $16 + 21 = 37$, and subtracting doesn't work here.