Concept:Certain plural nouns like "spectacles" (or "trousers", "scissors") refer to a single object made of two matching parts. We use "a pair of" to specify one such item.
Explanation:The word "spectacles" is always plural in form.
It denotes one eyeglass item consisting of two lenses.
To talk about one unit, we must say "a pair of spectacles".
Option (a) "a spectacle" is incorrect.
The word "spectacle" means a public event or display, not eyewear.
Option (b) "two spectacles" means two separate pairs, which is not the intended meaning.
The sentence says "bought spectacles" – meaning just one pair.
Option (c) "a pair of spectacles" correctly expresses one single eyewear item.
Thus the underlined part should be improved to "a pair of spectacles".
The improved sentence is: "Last evening I went to the optician and bought a pair of spectacles."
Since option (c) gives the best improvement, the correct answer is (c).
Answer:Option (c) – a pair of spectacles.