The bottleneck is the process that limits how many can be produced.
In washing, each dozen take 6 mins. The time available is 1,200 x 60 = 72,000 minutes. Therefore that process is capable of dealing with 72,000/6 = 12,000 dozen.
In filling , each dozen take 3 min. The time available is 700 x 60 = 42,000 minutes. Therefore that process is capable of dealing with 42,000/3 = 14,000 dozen.
Using the same approach, Capping can deal with 10,000 dozen, and Labelling can deal with 13,500.
So the most they can make is 10,000 dozen. It is capping that is stopping them making more and so capping is the bottleneck.