The correct option is (3).

\(E_{net} = \frac \lambda{4\pi \varepsilon_0r} (\sin \alpha + \sin \beta)\)
Here,
\(\lambda = \frac QL\), \(r = \frac{\sqrt 3}2 L\), \(\sin \alpha = \sin \beta = \frac 12\)
⇒ \(E_{net } = \frac{2Q}{4\pi \varepsilon_0\sqrt 3L^2} (\frac 12 + \frac 12)\)
⇒ \(E_{net } = \frac{Q}{2\sqrt 3\pi \varepsilon_0L^2}\)