What is a limitation of formulas that could be causing the issue?

Anapp builder wants to create a formula field on an Account to include data from related Contacts but is unable to find the relationship in the formula editor.

What is a limitation of formulas that could be causing the issue?
A . Unable to reference the child records.
B. A master-detail relationship should be created.
C. Formula field limit reached on the Account object.
D. More than 5,000 characters in the formula.

Answer: C