Here in the given numbers 91 is a Composite number. Since it has factors of 7 and 13 other than 1 and itself.
Composite Numbers :
A composite number is a positive integer which is not prime (i.e., which has factors other than 1 and itself).
Examples :: 4, 6, 8, 12, 14, 15, 18, 20, ...
They are units of measurement.
The knot is a unit of speed equal to one nautical mile (1.852 km) per hour, approximately.
The Watt is a unit of power.
A fathom is a unit of length in the imperial and the U.S. customary systems equal to 6 feet (1.8288 metres).
They are terms connected with black magic.
All these are the pieces in the game of chess.
They have steel plants.
They are famous for oil fields.
It is irrelavent
Clearly, though all the alternatives may form a part of the atmosphere, the air is the most vital part, without which there can be no atmosphere. So, the answer is (b).
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