Answer: Supply of T-shirts decreasing
Step-by-step explanation:
If the supply of T-shirts decreases, the equilibrium quantity of t-shirts being supplied to the market will decrease as well. Assuming that demand stays the same, the leftward shift of the supply curve will intersect with the demand curve at a higher equilibrium price.
This is simply because as the t-shirts are in short supply, people will be willing to pay more to have them as they are not as widespread as before.