Answer: a. paired samples;
Step-by-step explanation:
Paired samples are samples in which each data point in one sample is uniquely paired to a data point in the other sample.
Here, we have a paired sample of fecal boluses for gerbils by characterizing then as "No Drug" and "Drug".
hence, the design of this study is paired samples.
So, option A is correct.
NOTE : Independent samples are opposite of paired samples.
Testing the significance of a correlation require to check relation between two variables.