Select the option in which the numbers are related in the same way as are the
numbers in the following sets.

(NOTE: Operations should be performed on the whole numbers, without breaking down the numbers into its constituent digits. E.g. 13 - Operations on 13 such as adding /deleting /multiplying etc. to 13 can be performed. Breaking down 13 into 1 and 3 and then performing mathematical operations on 1 and 3 is not allowed)

(16, 18, 20)
(17, 19, 21)

The average of the first and third numbers in each set will form the second number.

(16, 18, 20)

second number = $$\frac{first\ number+third\ number}{2}$$

= $$\frac{16+20}{2}$$

(17, 19, 21)

second number = $$\frac{first\ number+third\ number}{2}$$

= $$\frac{17+21}{2}$$

= $$\frac{38}{2}$$

= 19

Option (a) (64, 66, 68)

second number = $$\frac{first\ number+third\ number}{2}$$

= $$\frac{64+68}{2}$$

= $$\frac{132}{2}$$

= 66

The above given logic is applicable here. So this is the correct answer.

Option (b) (24, 26, 30)

second number = $$\frac{first\ number+third\ number}{2}$$

= $$\frac{24+30}{2}$$

= $$\frac{54}{2}$$

= 27

The above given logic is not applicable here. So this is not the correct answer.

Option (c) (28, 30, 26)

second number = $$\frac{first\ number+third\ number}{2}$$

= $$\frac{28+26}{2}$$

= $$\frac{54}{2}$$

= 27

The above given logic is not applicable here. So this is not the correct answer.

Option (d) (36, 40, 42)

second number = $$\frac{first\ number+third\ number}{2}$$

= $$\frac{36+42}{2}$$

= $$\frac{78}{2}$$

= 39

The above given logic is not applicable here. So this is not the correct answer.