The correct answer is 180 ounces.
According to the information provided, the manager expects 120 people that night. The probability of ordering pasta is 1 in 5, which means that 24 of the 120 people visiting the restaurant will order pasta (120/5 = 24). Half of these people (12) are expected to order a dinner portion and half (12) will order the half portion. Since, the dinner portion weighs 10 ounces, 12 people will need 120 (12*10) ounces. Also, the half portion weighs 5 ounces, so 12 people will need 60 (12*5) ounces. In total, he will need 120+60=180 ounces of pasta.