Choose the correct word(s) that correctly completes the sentence. The family gave father a gold watch on the ____ of his fiftieth birthday.

Solution:

'Occasion' is the right word.
The term 'time' refers to a specific hour or day. For example- In my time that was an illegal act. The meaning of 'time' varies depending on the situation. As a result, it is the wrong option.
The term 'event' refers to a specific incident or subject. For example- My company has organized several events to invite foreign dealers. The meaning of 'event' varies depending on the context. As a result, it is the incorrect option.
The term 'occasion' refers to the occurrence of a specific moment or event. The meaning of 'occasion' is comparable to that of the provided context. As a result, it is the correct option.
Celebration refers to the action of honouring or remembering a significant event or day.For example- They organized a party to celebrate their sons birthday. Celebration is different in meaning to the given context.