Introduction to Biology: Scientific method.
A scientific instrument is a tool used to study phenomena or do theoretical research. They can be simple or very complex, it goes from scales, rules, and thermometers to spacial telescopes and DNA sequencers.
If we look at the answer options, we can identify the observations where an instrument is needed: The temperature of a glacier needs to be measured with a thermometer, the size of the virus requires the aid of a special microscope, and the color of a distant star can only be detected with a telescope. Only the number of dogs in a pack can be counted without an instrument.
This means B. the number of dogs in a pack is the right answer.