What is the simple subject of the following sentence?
during the night, the frightened child called for his mother and father.
a. child
b. frightened
c. his
d. night

Subject of a sentence is what the sentence is about: the object or thing that the sentence says something about.

Here a complete subject of the sentence:
"the frightened child"

The main word in the subject is "Child" and this is also the simple subject of the sentence: option A.