Assertion:Lithium (Li) has 1 valence electron.
Reason:The number of electrons in the outermost (valence) shell are called the valence electrons.
A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
C. Assertion is correct but Reason is incorrect
D. Both Assertion and Reason are incorrect

Hint:Initially one should know what is a valence electron and where they are found. This must be the first thought after reading the question. Now you should count the number of electrons present in the Lithium.

Complete step by step answer:
Valence electrons are nothing, but the electrons which are present in the outermost shell of any atom. These elections are the one which helps in forming bonds. If you take an example of a castle then you’ll easily understand it. The guard which is standing at the outermost door is solely responsible for fights. That guard is the valence electron in an atom.
Lithium is an element in the chemistry which has been assigned with an atomic number 3.
Let’s see electron distribution in lithium: \[{\mathbf{1}}{{\mathbf{s}}^{\mathbf{2}}},{\mathbf{2}}{{\mathbf{s}}^{\mathbf{1}}}\]
It is clearly visible that the last orbital contains only a single electron. Therefore, the lithium atom only contains a single valence electron or 1 valence electron.
So, clearly we can conclude that the correct answer is Option B.

Note:The only way to get the answer on your fingertips is to count the number of electrons and prepare a rough mind sketch of electron distribution.
Lithium is present in biological systems in huge amounts. Its functions are uncertain. Lithium has a single valence electron that is easily given up to form a cation. Because of this, lithium is a good conductor of heat and electricity as well as a highly reactive element, though it is the least reactive of the alkali metals. Lithium's low reactivity is because of the proximity of its last electron(valence electron) to its nucleus (the remaining two electrons are within the 1S orbital, much lower in energy, and do not participate in chemical bonds). However, molten lithium is significantly more reactive than its solid form.