correct answer is: a) 5

Explanation

Let the number which will add is KaTeX parse error: KaTeX doesn't work in quirks mode.
According to the question,
KaTeX parse error: KaTeX doesn't work in quirks mode.
KaTeX parse error: KaTeX doesn't work in quirks mode.
KaTeX parse error: KaTeX doesn't work in quirks mode.
KaTeX parse error: KaTeX doesn't work in quirks mode.
KaTeX parse error: KaTeX doesn't work in quirks mode.
KaTeX parse error: KaTeX doesn't work in quirks mode.
So, the number is KaTeX parse error: KaTeX doesn't work in quirks mode. which added to each term of the ratio KaTeX parse error: KaTeX doesn't work in quirks mode. to make the ratio KaTeX parse error: KaTeX doesn't work in quirks mode.
Ans: The number is KaTeX parse error: KaTeX doesn't work in quirks mode.