A vector is basically an arrow that tells you where to go. Specifically it tells you a direction and how far. The direction component is the angle. The "how far" component is the magnitude or modulus or length of the vector.
For choice C in the upper right hand corner, we see that the vector 'a' starts off on the right side and points directly to the left. It's a horizontal vector. The arrow is at the left side. After going from tail to tip along vector 'a', we repeat the same motions for vector b. Think of it as following directions on a map. To take a shortcut, we follow the resultant vector r. This takes us from the tail of vector 'a' to the tip of vector b without any obstacles or deviations. So this is why choice C is the answer.
Note: if you use the parallelogram law for choice A, then the resultant vector will be the same as shown in choice C. This is of course assuming that each vector 'a' is the same length and direction between the two answer choices. The same assumption is for the b vector as well. Choice D is close to what choice C is saying, but the arrow for r is pointed the wrong way. The arrow head must be pointed at the tip of vector b which is the final destination we want.
