あの 店は 午後六時に （ ）ます。
あの みせは ごごろくじに （ ）ます。
a. しまり b. しめ c. とざし d. とじ
The sentence says: “That shop closes at 6 pm,” rather than “somebody closes the shop at 6 pm.” So the correct answer is a. しまります, the intransitive option. If you thought it was b. then you need to say （[somebody]は）あの 店を 午後六時に しめます。
As are most verbs, しまる describes the change from the open status to the closed status, so if you want to say “the shop is closed (as a status)”, you need to say it as あの店は しまっている。
Other 3 verbs, 閉（し）める, 閉（と）ざす and 閉（と）じる are all transitive verbs, so you can say:
店を閉める
店を閉ざす
店を閉じる
However, the above 3 all can sound a lot more like “the shop is closing permanently” than when you say 店が閉（し）まる.
