What is the symbol used to designate the element potassium? 

One symbol is used to denote the element phosphorus, another to denote the element polonium, a fifth to denote the element protactinium, a sixth to denote the element sodium, and a seventh to denote the element potassium.

The English term potash is where the name potassium comes from. The term kalium, which is the Medieval Latin word for potash and may have sprung from the Arabic word qali, which denotes alkali, gives origin to the chemical symbol K. Potassium is a soft element with a metallic colour and belongs to the alkali group on the periodic table.

The element potassium is denoted by the symbol K of the following list.
The element phosphorous is denoted by the letter P in the list of alternatives below.
The element polonium is represented by the symbol Po among the following choices.
The element protactinium is denoted by the symbol Pa among the following choices.
The element sodium is denoted by the symbol Na among the following choices.

 The symbol used to designate the element potassium is $\mathrm{K}$.

Hence, option B is correct.