Final answer:
Contrastive sounds are those that can change the meaning of a word when one is substituted for the other, such as in minimal pairs like 'bat' and 'pat'.
Step-by-step explanation:
When two sounds are considered contrastive, it means option b) They can change meaning when substituted for one another. In the field of phonology, contrastive sounds are also known as minimal pairs: pairs of words or phrases in a particular language, that differ only by a single sound.
An example of contrastive sounds is the pair: bat and pat, where the substitution of /b/ for /p/ changes the meaning of the word.