The correct option is D narrows
The word 'broadens' refers to becoming larger in distance from side to side.
Option A, restricts refers to putting a limit on something.
Option B, contracts refers to decreasing in size, number or range.
Option C, shrinks refers to becoming or making smaller in size or amount.
Option D, narrows becoming or making less wide.
Thus, according to the meaning, the word 'narrows' is the opposite of the word 'broadens'.