Which of the following is least likely to behave as a Lewis base?

The periodic time of a particle doing simple harmonic motion is $4 \ s$. The time taken by it to go from its mean position to half the maximum displacement (amplitude) is

$A$ small source of sound moves on a circle as shown in the figure and an observer is standing at $O$. Let $n_1, n_2$ and $n_3$ be the frequencies heard when the source is at $A, B$ and $C$ respectively. Then

The nucleus of an element contains $9$ protons. Its valency would be

Let $P = \{ (x, y) | x^2 + y^2 = 1, x, y \in R \}$. Then $P$ is

Which of the following is logically equivalent to $\sim(\sim p \Rightarrow q)$?