Answer:
1/4 or 25%
Explanation:
The set of new shoes is composed by all pairs bought by either Harry or Kate.
Harry bought a pair of rain boots and a pair of tennis shoes while Kate got a pair of tennis shoes and sandals, totaling 4 pairs. Out of those 4 pairs, only 1 pair are rain boots, the fraction corresponding to rain boots is:

Rain boots are 1/4 of the set of new shoes.