The threshold on a dose-response curve is the:.
a. point at which the dose of the drug causes maximal response.
b. lowest dose at which there is an observable effect of the drug.
c. safe effective dose of the drug.
d. therapeutic index of the drug.

B. lowest dose at which there is an observable effect of the drug

b. lowest dose at which there is an observable effect of the drug.

Explanation:

If an  organism is exposed to a particular  dose(concentration) of drug or chemical for a period of time, and one needs to measure the extent or degree of exposure from  level of response of the organism to the dose,( to determine the potency, or adverse effect of the drug) the term   used to describe this procedure is   called dose-response relationship.

If it is possible to graph the the( log )dosage of the drug on( the x-axis) against the percentage response of the organism involved(on the y-axis), then it is possible to obtain a graphical representation of the magnitude of the response to the drug dosage in a Sigmoid  curve shape. This representation is called dose-response curve.

Generally in most cases small doses of drugs do not  produce toxic effects; however as dose increases, toxicity increases.However the minimum dose(drug concentration) at which toxicity first appears, that is where observable effects of the drug first noticed based on the response is called Threshold.

allowed for the prediction of the properties of missing elements.

d. organisms have cells with different shapes and functions.