Which of the following elements has the maximum density?

The highest oxidation state of $Cr$ is:

Assertion $(A)$: While going from left to right in the periodic table,the atomic size decreases more rapidly for the $3d$-series compared to the $4f$-series of elements.
Reason $(R)$: $3d$-electrons experience lesser shielding than $4f$-electrons.

Total number of $d$ electrons present in an element with atomic number $78$ is:

Identify the incorrect statement in the following.

To which block does the element with atomic number $106$ belong?