The correct answer is option 2: Potassium.
Soft soap is a type of soap that is made by combining potassium hydroxide with vegetable oils. Potassium hydroxide, also known as caustic potash, is a compound that contains the element potassium.
Options 1, 3, and 4 are all incorrect for this question. Caesium (option 1) is not typically found in soap production and is more commonly used in specialized applications such as atomic clocks. Calcium (option 3) and magnesium (option 4) are both elements that are important for living organisms, but they are not typically used in the production of soap.
In conclusion, the correct element found in soft soap is potassium, making option 2 the correct answer.