Firms shut down at a point where their Price<Minimum Average Variable Costs.
Now in a perfectly Competitive Market, Price Charged is always equal to Marginal Cost(MC). So, MC column above depicts the Price Charged at different quantities of the water bottles.
Now, MC Equals AVC when AVC is Minimum.
Thus, Minimum AVC = $3.50
Now if Price below 3.50, firm will shut down. Thus, firm's shut down point occurs at a quantity of 300 bottles of water per week and the market price is $3.50 a bottle.