Which of the following is true about lanthanides?

A. they are all radioactive
B. they are highly electropositive
C. they are very common elements
D. they have low melting and boiling points

B: they are highly electropositive

well, A and C are blandly not true. A quick check of physical properties https://en.wikipedia.org/wiki/Lanthanide#Physical_properties_of_the_elements shows D as not true. So....

E. They like to wear tiny hats and tell joke-lanthides.

A. they are all radioactive

To determine which statement about lanthanides is true, let's analyze each option:

A. They are all radioactive: To determine the accuracy of this statement, you can search for information regarding the radioactive properties of lanthanides. One way to find this information is by referring to reliable sources, such as scientific journals or reputable websites dedicated to chemistry or the periodic table.

B. They are highly electropositive: To determine the accuracy of this statement, you can search for information regarding the electropositivity of lanthanides. This information can typically be found in chemistry textbooks, scientific articles, or online resources.

C. They are very common elements: To determine the accuracy of this statement, you can search for information about the abundance of lanthanides in nature. This information can be found in geology or geoscience references, as well as scientific databases or encyclopedias.

D. They have low melting and boiling points: To determine the accuracy of this statement, you can search for information regarding the melting and boiling points of lanthanides. This information can be found in chemistry references, textbooks, or online databases that provide data on various elements.

By researching each of the statements provided using reliable sources, you can identify which one accurately describes the properties of lanthanides. Remember to critically assess the information and cross-reference it with multiple sources to ensure accuracy.