Catch at a straw means to try any small hope when desperate. The correct answer is this meaning because the expression is used figuratively, not only by its literal words.

Examples:

• The failing company began to catch at a straw.
• In an HSC essay, Catch at a straw can express this meaning: to try any small hope when desperate.
• When this expression appears in a passage, connect Catch at a straw with the idea of to try any small hope when desperate.