1 - a (join - fatty foods)
2 - d (cut down on - weight)
3 - b (lose - a diet)
4 - c (go on - a gym)
Speech bubbles:
A: I should cut down on and start eating more fruit and vegetables.
B: I'm glad I go on now I can exercise more often.
C: This is my last slice of pizza! I'll lose tomorrow!
D: Congratulations, you join about 3 kilos!