Fill in the blank with the most appropriate word.They _________ basketball when I saw them.

Answer: Option A

The correct answer is Option A: were playing. In this sentence, the past continuous tense "were playing" is appropriate because it indicates an ongoing action in the past. The phrase "when I saw them" indicates that the action was in progress at a specific point in the past.
The other options are not correct:
B. "played" (Incorrect) - This option uses the simple past tense, which does not convey the ongoing nature of the action.
C. "play" (Incorrect) - This option uses the simple present tense, which is not suitable for describing an action that was happening at a specific point in the past.
D. "are playing" (Incorrect) - This option uses the present continuous tense, which is not appropriate for describing a past action.