ANSWER
[tex] \sin(x) = \frac{a}{b} [/tex]
EXPLANATION
It was given that,
[tex] \tan(x) = \frac{a}{4} [/tex]
and
[tex] \cos(x) = \frac{4}{b} [/tex]
We can use the relation,
[tex] \tan(x) = \frac{ \sin(x) }{ \cos(x) } [/tex]
This implies that,
[tex] \cos(x) \times \tan(x) = \sin(x) [/tex]
We now substitute the given values to get,
[tex] \frac{a}{4} \times \frac{4}{b} = \sin(x) [/tex]
[tex] \sin(x) = \frac{a}{b} [/tex]
Or
Since
[tex] \tan(x) = \frac{a}{4} = \frac{opposite}{adjacent} [/tex]
It means the length of the opposite side is
[tex]a \: units[/tex]
and the length of the adjacent side is
[tex]4 \: units[/tex]
Also,
[tex] \cos(x) = \frac{4}{b} = \frac{adjacent}{hypotenuse} [/tex]
This also means that, the hypotenuse is
[tex]b \: units[/tex]
But
[tex] \sin(x) = \frac{opposite}{hypotenuse} [/tex]
This implies that,
[tex] \sin(x) = \frac{a}{b} [/tex]
The correct answer is D.