Writing in percentages is actually writing a fraction with 100 on its denominator.
By doing so, we can easily compare data.
For example 1/2 may be written as 50/100 and it is denoted by 50%.
So, in your question, 77% means 77/100.
Since fractions represent ratios, writing in ratio form, we get 77:100. The answer is (b)77:100
