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Choice B is correct. It’s given that in a group, $40\%$ of the items are red. It follows that the number of red items in the group can be represented by $0.4x$, where $x$ represents the total number of items in the group. It’s also given that of all the red items in the group, $30\%$ also have stripes. It follows that the number of items in the group that are red and have stripes can be represented by $0.3\left(0.4x\right)$, or $0.12x$. The expression $0.12x$ represents $12\%$ of $x$. Since $x$ represents the total number of items in the group, it follows that $12\%$ of the items in the group are red and have stripes.

Choice A is incorrect and may result from subtracting $30\%$ from $40\%$ rather than calculating $30\%$ of $40\%$.

Choice C is incorrect and may result from adding $30\%$ and $40\%$ rather than calculating $30\%$ of $40\%$.

Choice D is incorrect and may result from calculating the percentage that $30\%$ is of $40\%$ rather than calculating $30\%$ of $40\%$.