What change should be made in sentence 20?
a. add a comma after although
b. delete the comma after different
c. change task to task's
d. change was surprisingly to were surprisingly.

The correct answer is D because there are multiple tasks

The answer is D.

Although the animals were different, the daily tasks of each zookeeper were
surprisingly similar. 

Reasoning for this would be if it already said: the animals were different, than you would want to change the was into were too.