A false positive result is best described as one that is given

The correct option is "by a substance other than that being tested for".

Step-by-Step Explanation:

1. Understanding a False Positive:
In scientific testing, diagnostics, and chemical assays, a "positive" result indicates that the test detected the target substance. A "false positive" occurs when the test incorrectly indicates the presence of that target substance, even though it is not actually there in the sample.

2. Why does this happen?
Many chemical and biochemical tests rely on reactions (such as color changes, precipitation, or binding) that are designed to be specific to a target substance. However, if another substance is present in the sample that has a similar structure or chemical property, it can react with the test reagents in the same way. This is known as cross-reactivity.

3. Analyzing the Options:
- When a substance is present in large or minute quantities, a positive result is expected (or might be missed, which is a false negative), but it is not a "false" positive because the target substance is indeed present.
- A false positive is specifically triggered by a substance other than that being tested for because that interfering substance mimics the target analyte and causes the test to generate a positive signal falsely.