The correct sentence to be formed in the above case is with “number” because tests are pronouns should be represented by number to produce grade for students.
This is done by the professor. The word” amount (represents statistical calculation)”, “stacks (represents bunch)” etc cannot be used in the above case as they may from grammatically incorrect sentence formation.
Thus, the options (c), (b) and (d) are incorrect.
The correct sentence to be formed in the above case is with “number” because tests are pronouns should be represented by number to produce grade for students.
Therefore, the correct option is – a. number.