Awave is traveling through a particular medium. the wave source is then modified so that it now emits waves at a higher frequency. does the wavelength a.)increase b.) decrease c.) stay the same d.) depend on other factors

The correct answer is
B.) decrease 

In fact, the relationship between frequency, wavelength and speed of a wave is given by

where
is the wavelength
f is the frequency
v is the speed of the wave

The speed v of a wave depends only on the properties of the medium; in this problem the medium does not change, so the speed remains the same. 
Instead, the frequency f increases: from the previous equation we see that the wavelength is inversely proportional to the frequency, therefore if the frequency increases, the wavelength will decrease.