If a basalt body cuts across a fault, what are the relative ages of the basalt and the fault?
a. the fault must be older, according to the principle of cross-cutting relationships.
b. the basalt must be older, according to the principle of cross-cutting relationships.
c. the basalt must be older, according to the principle of original horizontality.
d. their relative ages cannot be determined from the information given.

Option (B)

Explanation:

According to the law of cross-cutting relationships, in a geological feature where an igneous rock body is intruded by any intrusive body such as a fault and d(i)kes, then the intrusive bodies are considered to be younger than the surrounding igneous rock.

In the given condition, the basalt is the older surrounding rock which is being cut across by a fault, and this follows the principle of cross-cutting relationship.

Thus, the correct answer is option (B).