The simple sentence is c. animal testing is a cruel way to advance the progress of science.
You can classify simple and complex sentences by the number of verbs in it
You can see, in the other sentences, that they have two conjugated verbs and they have words like "because", "and" it's like they are giving more than one information:
a.because animal testing is so cruel, it should be stopped immediately.
b. the practice of animal testing should be stopped and the labs dismantled.
c.animal testing is a cruel way to advance the progress of science.
d.animal testing is cruel because animals suffer horrible deaths.
