Recall the definition of lanthanoids
Lanthanoids are the elements with atomic numbers from 58 to 71, i.e. the elements following lanthanum in which the $4f$ subshell is progressively filled. Commonly included are:
$\mathrm{Ce}, \mathrm{Pr}, \mathrm{Nd}, \mathrm{Pm}, \mathrm{Sm}, \mathrm{Eu}, \mathrm{Gd}, \mathrm{Tb}, \mathrm{Dy}, \mathrm{Ho}, \mathrm{Er}, \mathrm{Tm}, \mathrm{Yb}, \mathrm{Lu}$

Check each given element

• $\mathrm{Eu}$ (Europium): lanthanoid
• $\mathrm{Cm}$ (Curium): not a lanthanoid; it is an actinoid
• $\mathrm{Er}$ (Erbium): lanthanoid
• $\mathrm{Tb}$ (Terbium): lanthanoid
• $\mathrm{Yb}$ (Ytterbium): lanthanoid
• $\mathrm{Lu}$ (Lutetium): lanthanoid

Count the elements that are not lanthanoids

Only $\mathrm{Cm}$ is not a lanthanoid.

Therefore, the number of elements that do not belong to lanthanoids is $1$

Match with the options

Option B: 1 is correct.