Assertion: In some solids, the valence band is completely filled while the next higher band separated by an energy gap of a few electrons-volts is completely empty.
Reason: In insulators the energy band completely filled with electrons (the valence band) and above it is empty band (the conduction band)
A. Both, Assertion and Reason, are true and Reason is the correct explanation of Assertion.
B. Both, Assertion and Reason, are true but Reason is not the correct explanation of Assertion.
C. Assertion is true but Reason is false.
D. Assertion is false but Reason is true.

Hint: In this type of question, we have to check for the statement given and the reason for the given statement are right or wrong and also for the correct explanation. To solve, we use the concept of valence band theory of the solids which states that energy bands in solids are separated by an energy gap called gas forbidden gap or band gap.

Complete answer:
According to the band theory of solids, closely spaced energy levels of all electrons in a particular orbit is called an energy band. The electrical properties of material depend upon the number of electrons in the outermost orbit. These electrons are valence electrons and the corresponding band is called valence band.

The free electrons in the conduction level correspond to the conduction band. The energy gap between valence and conduction band is called forbidden gap or band gap.

Now, the solids are classified into three subcategories as – metals, insulators and semiconductors. Metals having both the valence and conduction bands partially filled and the energy band gap is very small. But, for insulators. Valence band is completely filled and conduction band is empty. Also, the band gap is of the order of 6 electron-volts. Hence, In the given question, assertion and reason are true and Reason is the correct explanation of assertion.

Hence, option A is correct.

Note: In insulators the energy band is completely filled with electrons (the valence band) and above it is empty band (the conduction band), this is because there are no any free electrons in the conduction band and hence, the conductivity is zero.