Answer is: C. bias or judgment.
Objective observations are accurate if there are not influenced by the observer's judgenebt, feelings, opinions or biases.
Observation is the active purchase of information from a primary source.
There are two types of data collecting: quantitative and qualitative observations.
1) Quantitative observations are made with instruments (in this example balance).
This data are measurable and can be expresed with numbers.
2) Qualitative observations are information about the quality of substances.
In qualitative observation, informations are collected using senses (sight, hearing, taste, smell and touch).
Qualitative observations are not measurable and cannot be recorded in terms of numbers.