Which of the following fractions represents the value of the given model?

Both squares in the given model are divided into 8 equal parts.

The first block has 1 shaded part and hence represents the fraction $\frac{1}{8}$.

The second block has 5 shaded parts and hence represents the fraction $\frac{5}{8}$.

Now, the sum of fraction $\frac{1}{8}$ and $\frac{5}{8}$ = $\frac{1}{8} + \frac{5}{8} = \frac{1 + 5}{8} = \frac{6}{8}$.

The two squares can be joined as:

Thus, the resultant square is also divided into 8 parts, out of which, 6 parts are shaded. 

Thus, the resultant square represents the fraction $\frac{6}{8}=\frac{3}{4}$.