The answer to your question would be that the personal pronoun that completes the sentence is the following one: Who.
Explanation:
The sentence above is an example of a nonessential relative clause. You can tell this by looking at the punctuation: The type of clause introduced by the missing personal pronoun is a nonessential one as it is separated from the rest of the sentence by means of the comma. What is more, you use the pronoun <em>who</em> to refer to people and it can be either the subject or object of the clause. <em>Whose</em> is used to indicate possession, <em>whom</em> as the object of a preposition or verb and <em>whoever</em> is an indefinite relative pronoun.
because through the process of convection and conduction, the heat is transferred from the water to the metal handle. convection is the movement of heat through fluid, conduction is heat moving to the handle.