Recall that the tangent of an angle is equivalent to the ratio:
Recall also that a fraction is undefined if the expression in the denominator is zero.
This implies that if the value of the cosine function in the denominator equals zero, then the tangent becomes undefined.
Hence, to find which value of Θ will make tan Θ undefined, it is required to find the value of Θ for which cos Θ is 0.
It is known that cos 270º=0. The cosine of other values will not give zero.
Therefore, the value of Θ for which tanΘ is undefined is Θ=270º.
The correct choice is 270º.