The mother was anxious . . . . . . . . the safety of her son. A. upon B. about C. at D. for

The correct answer is: B. about

The word “about” is used to express concern or worry about something. In this case, the mother is worried about the safety of her son.

The other options are not as appropriate in this context.

• The word “upon” is used to express a feeling of responsibility or obligation. For example, “The king was upon his throne.”
• The word “at” is used to express a feeling of anger or annoyance. For example, “The man was angry at the dog.”
• The word “for” is used to express a feeling of desire or longing. For example, “The woman was longing for her husband.”

I hope this helps!