correct answer is: a) 1\;:\;3

Explanation

Here, given ratio is 2\;:\;3,\;5\;:\;4,\;6\;:\;15\;.
Or, \large\frac{2}{3},\frac{5}{4},\frac{6}{15}
To find the compound ratio, we use the following method :
\rightarrow\frac{Multiple\ of\ all\ Numeretors}{Multiple\ of\ all\ Denominators}
\rightarrow\large\frac{2\times5\times6}{3\times4\times15}
\rightarrow\large\frac{1}{3}
So that, the compound ratio is 1\;:\;3\;.
Ans: 1\;:\;3\;.

An alternative way to phrase this particular math query is possible:

1. If we combine the ratios 2 : 3, 5 : 4, and 6 : 15, what is the compound ratio?
2. Calculate the compound ratio of the ratios 2 : 3, 5 : 4, and 6 : 15.
3. Given the ratios 2 : 3, 5 : 4, and 6 : 15, what is their compound ratio?