Fill in the blank in the given sentence by choosing the correct option. The…

Concept

"lest" is a subordinating conjunction that introduces a clause naming an undesirable outcome the subject wants to avoid. That clause customarily takes the modal "should" (a fixed usage pattern, "lest ... should"), and the whole construction carries the sense of "for fear that" or "so that ... would not".

Application

Here, the old woman's caution while carrying her heavy load is explained by the outcome she wants to avoid, namely falling. The blank needs the reason/purpose conjunction that pairs with "should" to name that avoided outcome, giving: "The old woman walked carefully with her heavy load lest she should fall."

Why not the other options

• "unless" introduces a condition of exception ("except if"); using it here would wrongly suggest her caution applied only in the absence of falling, reversing the intended cause and effect.

• "until" marks a point in time up to which an action continues; using it here would mean she was careful only up to the moment of falling, not that her caution existed to avoid falling.

• "although" introduces a concession between two contrasting facts, but there is no contradiction in this sentence to concede - it needs a reason for her caution, not a concession.

"lest" is the pairing that correctly names the avoided outcome, giving the grammatically and logically correct sentence above. Correct sentence: "The old woman walked carefully with her heavy load lest she should fall."