Please correct me if i am wrong.
Let Ka -> kannada , Ta -> tamil and Te -> telegu.
n(Ka $\cup$ Ta $\cup$ Te ) = n(Ka) + n(Ta) + n(Te) - n(Ka $\cap$ Ta ) - n(Ka $\cap$ Te ) - n(Te $\cap$ Ta ) + n(Ka $\cap$Ta $\cap$ Te )
$\Rightarrow$ 50 = 24 +24 + n(Te) - 5 - 6 - 7 + 4
$\Rightarrow$ 50 = 48 + n(Te) - 14
$\Rightarrow$ n(Te) = 2 + 14 =16.
$\therefore$ people who know telegu n(Te) =16
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SO people who know only telegu
= n(Te) - n(Ka $\cap$ Te ) - n(Te $\cap$ Ta ) + n(Ka $\cap$Ta $\cap$ Te )
= 16 -6 -7 +4
= 7
7 is not in the option.