We can use a strategy known as factoring in parts

Given: jm + kn + km + jn = 12
Rearrange as follows: jm + km + jn + kn = 12
Factor in parts as follows: m(j + k) + n(j + k) = 12

Notice that both parts contain (j + k), so we can combine these terms in the same way we would take 3x + 2x and combine the terms to get (3 + 2)x, which is the same as 5x.

When we combine the terms we get: (m + n)(j + k) = 12

Also given: j + k = 4
So we can replace j + k with 4 to get: (m + n)(4) = 12
Divide both sides of the equation by 4 to get: m + n = 3

We now have:
Quantity A: 4
Quantity B: 3

Answer: A