Answer
(a) Exact
(b) Measured
(c) Measured
(d) Measured
Work Step by Step
(a) The number of students in the laboratory has to be exact, since it is impossible to have half a student or any fraction of a person.
(b) This number is clearly estimated, and the time between two events is usually measured/estimated.
(c) The mass of an object is always measured/estimated.
(d) The mass of cholesterol in 1 dL of blood had to be measured. (directly or indirectly).
