Answer:

In such questions you are supposed to look out for the pattern of the series.

As such, for first one :
7, 14, 28 ,35, ...  ; Here as you can tell the pattern is the multiple of 7. We are supposed to represent it for nth term..
We know that these are multiples of 7 and hence can be expressed as :

(7 × 1), (7 × 2), (7 × 3), (7 × 4), (7 × 5), (7 × 6)..., (7 × n)
Where value of n can be any natural number!

Hence the answers :
(a) 7, 14, 28, 35 , ... is 7n ; multiples of 7
(b) 9, 19, 29, 39, 49, .. is (10n + 9), as 9 is added to multiples of 10
(c) 5, 10, 15, 25, .... is 5n ; again multiples of 5
(d) 2, 4, 6, 8, 10 ... is 2n ; multiples of 2

Hope it Helps!

-A