To means to draw a conclusion based on something you observe.
a. guess
c. model
b. control
d. infer

D. inference

Observation is acknowledging and noting some facts from the surroundings or the environment. Usually, observation is done using the five senses of human such as the sense of sight, sense of smell, sense of touch, sense of hearing and sense of tasting. It can also be qualitative and quantitative. Qualitative observations are those who use non-numeric forms in expressing the observation however quantitative uses numeric forms in presenting the observation. Inference on the other hand is the probable explanation or interpretation based on the observation given. Therefore, the statement "The container is filled to the 350 ml mark with water " is an observation.