Explanation

‘Deplore’ means to feel or express strong disapproval, grief, or regret about something — to lament it. Of the options, ‘regret’ (to feel sorry about) is the nearest synonym. ‘Lose heart’ means to become discouraged, ‘entreat’ means to beg earnestly, and ‘malign’ means to speak evil of someone. None of these matches ‘deplore’. Hence (c) is correct.