Solution
$$\cosec2\theta = \sec(3\theta - 15)$$
=$$ \sec(3\theta +90 - 105)$$
= $$\sec(90 - (-3\theta + 105)$$
{$$sec(90 - \theta) = cosec\theta$$}
$$\cosec2\theta = \cosec(-3\theta + 105)$$
$$2\theta = -3\theta + 105$$
$$\theta = 21$$
So , the answer would be option d)$$21^\circ$$.
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