Question

There are 3 black marbles and 4 red marbles in a bag.

Answer 1

3 + 4 = 7 all marables in bag
The probability that the first marable is black
$P(A_1)=\dfrac{3}{7}$
The probability that the second marable is black too
$P(A_2)=\dfrac{2}{6}=\dfrac{1}{3}$
Together:
$P(A)=P(A_1)\cdot P(A_2)=\dfrac{3}{7}\cdot\dfrac{1}{3}=\dfrac{1}{7}\to D$