Choose the appropriate word which can be substituted for the given sentence: Study of birds.

Choose the appropriate word which can be substituted for the given sentence: 'Something which is found all over the world.'

Choose the appropriate word which can be substituted for the given sentence: '$A$ person sent on an official mission'.

Choose the appropriate word which can be substituted for the given sentence: 'The part of government which is concerned with making of rules'.

Choose the appropriate word which can be substituted for the given sentence: 'Extreme old age when a man behaves like a fool'.

Choose the appropriate word which can be substituted for the given sentence: '$A$ man who always thinks of himself'.