Assertion (A) : A bag contains red, white and blue pencils. The probability of selecting a red pencil is $\dfrac{2}{13}$ and that of selecting a blue pencil is $\dfrac{4}{13}$. Then the probability of selecting a white pencil will be $\dfrac{7}{13}$.

Reason (R) : The probability of all possible outcomes of an experiment must add upto 1.

1. Both A and R are correct, and R is the correct explanation for A.

2. Both A and R are incorrect.

3. A is true, but R is false.

4. A is false, but R is true.

Given,

The probability of selecting a red pencil = $\dfrac{2}{13}$

The probability of selecting a blue pencil = $\dfrac{4}{13}$

We know that,

The probability of all possible outcomes of an experiment must add upto 1.

So, reason (R) is true.

⇒ P(red) + P(blue) + P(white) = 1

Let probability of selecting a white ball be x.

$\Rightarrow \dfrac{2}{13} + \dfrac{4}{13} + x = 1 \\[1em] \Rightarrow \dfrac{6}{13} + x = 1 \\[1em] \Rightarrow x = 1 - \dfrac{6}{13} \\[1em] \Rightarrow x = \dfrac{13}{13} - \dfrac{6}{13} \\[1em] \Rightarrow x = \dfrac{13 - 6}{13} \\[1em] \Rightarrow x = \dfrac{7}{13}.$

So, assertion (A) is true.

∴ Both A and R are correct, and R is the correct explanation for A.

Hence, option 1 is the correct option.