Answer: E. and Electroweak in 1961 the only other, as
Explanation:
This is more an English grammar question than a physics question, so taking that perspective, one should look for the answer that best completes the sentence.
Based on the word "first" in the sentence, implying the need for a conjunction to join the two theories and the last part of the sentence does not give a reason but further supports the determination of the theory being the first unified theory.
So, to complete the sentence, the best option is;
Maxwell’s theory of Electromagnetism in 1865 was the first "unified field theory" and Electroweak in 1961 the only other, as no further theory has united the electroweak field with either the Strong (Hadronic) force or Gravity.
