Explanation:

- The original sentence mentions two simultaneous past events: the father's return from work and playing in the garden.

- Option 1: "am playing" is incorrect. It uses present continuous tense, which doesn't fit with the past event of the father returning from work.

- Option 2: "No improvement" implies the original sentence is correct as is, but the original sentence mixes past continuous and simple past events without conveying simultaneity well.

- Option 3: "play" uses simple present tense, which doesn't match the past context.

- Option 4: "was playing" uses past continuous tense, showing that the playing was ongoing when the father came home, which fits well.

The most natural choice is:

- "was playing"