Đáp án + Giải thích các bước giải:

Xét tứ giác DKMN có:
$\widehat{DKM} = \widehat{MND} = \widehat{NDK} = 90^o$

$\Rightarrow DKMN$ là hình chữ nhật.
$\color{#FF4500}{A}\color{#FF5E00}{u}\color{#FF751A}{r}\color{#FF8C33}{e}\color{#FFA34D}{l}\color{#FFBA66}{l}\color{#FFD180}{i}\color{#FFE899}{a}\quad\color{#FDC6FF}{V}\color{#F5A8FF}{i}\color{#EB89FF}{r}\color{#E06BFF}{e}\color{#D44DFF}{l}\color{#C72FFF}{y}\color{#BA11FF}{s}\quad\color{#A264F6}{C}\color{#8C7EF9}{a}\color{#7798FB}{e}\color{#61B2FE}{l}\color{#4BCCFF}{y}\color{#36E6FF}{d}\color{#20FFFF}{r}\color{#0DF5E6}{a}\quad\color{#6DE1F3}{A}\color{#84C3F1}{n}\color{#9AA6EF}{a}\color{#B189EC}{t}\color{#C76CEB}{h}\color{#DE4FE8}{a}\color{#F432E6}{r}\color{#FF1DE6}{i}\color{#FF00E6}{e}\color{#E600D1}{l}$