When two or more waves meet up while traveling through the same medium, they undergo interference. The interference that occurs can be classified as either  constructive or destructive interference.
 

You have to make two decisions to be successful with this question. You have to make a Wavelength decision and you have to make an Amplitude decision.

Wavelength Decision
You can use the background grid to determine the wavelength of the wave. You can count the squares. It can be thought of as the distance between one crest and the next adjacent crest. Once you determine the wavelength in the given diagram, look for an answer option that shows one-half that value for wavelength. You can do the square counting again to be sure. But in the end, you are looking for less stretched out wave; the repeating unit will repeat from often over the length of the diagram.

Amplitude Decision
You can use the background grid to determine the amplitude of the wave. You can count the squares. It can be thought of as the vertical distance from the resting position (the mid-line on the diagram) to the crest. Once you determine the amplitude in the given diagram, look for an answer option that shows one-half that value for amplitude. You can count squares again to be sure. But in the end, you are looking for a shorter wave.

These two decisions should allow you to narrow the answer options down to one.