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(C) The torque is given by $\vec{	au} = \vec{r} 	imes \vec{F}$.For option $A$: $\vec{r} = b \hat{j} - c \hat{k}$ and $\vec{F} = a \hat{k}$.$\vec{	a

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$F = a \hat{j}$,$P$ is $(-b, 0, -c)$

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(C) The torque is given by $\vec{	au} = \vec{r} 	imes \vec{F}$.For option $A$: $\vec{r} = b \hat{j} - c \hat{k}$ and $\vec{F} = a \hat{k}$.$\vec{	au} = (b \hat{j} - c \hat{k}) 	imes (a \hat{k}) = ab(\hat{j} 	imes \hat{k}) - ac(\hat{k} 	imes \hat{k}) = ab \hat{i}$. The $z$-component is $0$.For option $B$: $\vec{r} = -b \hat{j} - c \hat{k}$ and $\vec{F} = -a \hat{k}$.$\vec{	au} = (-b \hat{j} - c \hat{k}) 	imes (-a \hat{k}) = ab(\hat{j} 	imes \hat{k}) + ac(\hat{k} 	imes \hat{k}) = ab \hat{i}$. The $z$-component is $0$.For option $C$: $\vec{r} = -b \hat{i} - c \hat{k}$ and $\vec{F} = a \hat{j}$.$\vec{	au} = (-b \hat{i} - c \hat{k}) 	imes (a \hat{j}) = -ab(\hat{i} 	imes \hat{j}) - ac(\hat{k} 	imes \hat{j}) = -ab \hat{k} - ac(-\hat{i}) = ac \hat{i} - ab \hat{k}$.The $z$-component is $-ab$,which is negative since $a, b > 0$.

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