The given sentence states an action that happened in the past. Also, the action is a continuing at that particular point of time in the past.
Option A: The past continuous tense also indicates that an action was continuing at a point of time in the past.
'Were playing' is in the past continuous tense, as it follows the structure of a verb in this tense: was/were + past participle (playing).
Hence option A is correct.
Option B: The past perfect tense (had played) speaks of an action that completed in the past, before another action commenced. But, in this sentence, no other action is mentioned.
Option C: The simple past tense is used when the action has completed in the past. Also, its form is: played. This isn't the form here.
Option D: Present perfect tense indicates an action that has just completed in the present. But, the action here is still continuing in the past.
Hence options B, C and D are incorrect.