The word 'lengthen' refers to making or becoming longer.
Option A, protract refers to extending the duration of something.
Option B, brighten refers to lightening up of something.
Option C, abandon refers to deserting someone or cease to support or look after something.
Option D, shorten refers to reducing or lessening in size.
Thus, according to the meaning, the word 'shorten' is the most opposite in meaning from the word 'lengthen'.