It is just a question of identifying what are $y_i$ and $u_i$. The force $y_i$ is proportional to the applied mass (3rd column in the table). The displacement $u_i$ is the amount the spring is extended **from the equilibrium position**. The equilibrium position is when the force (mass) is $0$. That means looking at the second column and subtracting $0.140$ from the current position. Let's call our matrix $M$, with elements $M_{ij}$, with $i$ from $1$ to $5$ and $j=1$ or $j=2$. When we do the matrix-vector multiplication, we can write $y_1=M_{1,1}b+M_{1,2}k$. We know that $y_1=b+k u_1= b+k 0$. This means $M_{1,1}=1$ and $M_{1,2}=0$. At this point, the possible answers are B. or C. If you write the same equations for the second term you get $M_{2,1}b+M_{2,2}k=b+0.051k$, so the answer is C.