Step 1: Understanding the Question.
This question involves identifying the type of proteins found in the blood that interact with enzymes.
Step 2: Analyzing the Options.
(A) Soluble proteins: Soluble proteins are present in blood but don't necessarily interact with enzymes in this context.
(B) Complex proteins: Complex proteins may interact with enzymes but are not the primary answer here.
(C) Inert proteins: Inert proteins do not interact actively with enzymes.
(D) Functional proteins: Correct — Functional proteins, such as enzymes and hormones, interact with other molecules in the body to carry out various functions.
Step 3: Conclusion.
The correct answer is (D) Functional proteins, as they are responsible for various interactions in the body, including with enzymes.