A subject complement is the noun, pronoun or adjective, following linking verb.
True linking verbs are the following verbs- be, become and seem. These three verbs are linking verbs always.
Also, there are several verbs which can be action or linking-feel, appear, remain, taste, turn etc.
Therefore, a sentence which contains a subject complement is,
C. Our city's most pressing problem today is traffic.
Herein, 'is' is the linking verb of the form 'be' and 'traffic' is the subject complement as it describes what the problem is. Also, the linking verb 'is' makes it easier to identify.
