The figures below show regular hexagons with charges at the vertices. In which of the following cases is the electric field at the center $NOT$ zero?

Charges $Q_{1}$ and $Q_{2}$ are at points $A$ and $B$ of a right-angled triangle $OAB$ (see figure). If the resultant electric field at point $O$ is perpendicular to the hypotenuse $AB$,then $Q_{1} / Q_{2}$ is proportional to:

An observer at rest experiences an electric field due to a static charge. Later,he starts moving away from it. What does the observer experience?

Two charges $Q$ and $4Q$ are separated by a distance of $6 \text{ cm}$. The distance of the point from $4Q$ at which the net electric field is zero is: (in $\text{ cm}$)

$A$ unit negative charge with mass $M$ resides at the mid-point of the straight line of length $2a$ connecting two fixed charges of magnitude $+Q$ each. If it is given a very small displacement $x$ $(x \ll a)$ in a direction perpendicular to the straight line, it will:

An infinite number of electric charges, each equal to $5 \text{ nC}$ (magnitude), are placed along the $X$-axis at $x = 1 \text{ cm}, x = 2 \text{ cm}, x = 4 \text{ cm}, x = 8 \text{ cm}, \dots$ and so on. In this setup, if the consecutive charges have opposite signs, then the electric field in $\text{N/C}$ at $x = 0$ is: $\left(\frac{1}{4 \pi \varepsilon_0} = 9 \times 10^9 \text{ N} \cdot \text{m}^2/\text{C}^2\right)$