The correct option is B Swat
'To slap with a flat object' is called Option B, Swat. Option A, 'Hew' means to cut or chop with an axe or some other tool. Option C, 'Chop' means to cut something to pieces. Option D, 'Gnaw' means bite or chew. Thus, options A, C and D are incorrect and the correct word for the given phrase is Option B, Swat.