Understanding the Concept:
A transition element must possess partially filled \(d\)-orbitals in its atom or in at least one oxidation state.
Step 1: Electronic configuration of mercury.
Atomic number of mercury:
\[
Z=80
\]
Electronic configuration:
\[
[Xe]4f^{14}5d^{10}6s^2
\]
Its \(5d\)-subshell is completely filled.
Step 2: Common oxidation state of mercury.
Mercury commonly forms:
\[
Hg^{2+}
\]
Electronic configuration of \(Hg^{2+}\):
\[
[Xe]4f^{14}5d^{10}
\]
Again, the \(d\)-subshell remains completely filled.
Therefore, mercury does not satisfy the definition of a transition element.
Hence, Assertion is true.
Step 3: Analyzing the reason.
Mercury is indeed a liquid metal at room temperature.
However, being liquid has no relation to the definition of transition elements.
Thus, the reason is true but it does not explain the assertion.
Therefore, option (2) is correct.