Answer:
D. It is extremely reactive.
Explanation:
Potassium has atomic number of 19 and belongs to the group 1 elements.
A. It is dull and brittle
This is not true as potassium is a metal with a silver lustre.
B. It is not reactive
This is false. Because it is a group 1 element, it would readily lose it's electron to achieve octet configuration making it a reactive metal.
C. It is white and hard.
Potassium is a white metal, however it is not hard. Rather it is soft.
D. It is extremely reactive.
This is correct. Because it is a group 1 element, it would readily lose it's electron to achieve octet configuration making it a reactive metal.
