Assertion: Boron forms only covalent compounds.
Because: Due to the small size of boron the sum of its first three ionizations are very high.
(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 the reason is incorrect
(D) Assertion is incorrect but reason is correct

Hint:In order to solve this question we need to know that the size of boron is very small so its electrons will be very close to the center due to which there will be a large amount of energy required to eject all its electrons. So it only forms the covalent compounds.

Complete step-by-step answer:So for solving this we need to have the knowledge about boron hence we will learn it first,Boron is the chemical element with symbol B and atomic number 5. Which produces the cosmic ray spallation and supernovae and not by stellar nucleosynthesis, it is a low abundance element in the solar system and in the earth crust. Its atomic mass is 10.811 u, and it has a high electronegativity of 2.04. Since the atomic number is 5 so the electronic configuration will be:
He - $1{s^2}2{s^2}2{p^1}$
And its van der waal radius is 180 pm.Since we have seen that it has a small radius so the electrons present in the B are very close to the nucleus, a result these are strongly attracted by the nucleus which needs a large amount of energy to eject and form the ionic bond so this process is very typical.In a case one electron may be ejected so the resultant positive charge will be greater than the negative charge so it will be more difficult to make the ejection of second electron and its ionization energy will be more than as compared to when the first electrons ionization energy.Now for the last one there will +2 net charge on the atom so it will provide much barrier to eject the last electron so the ionization energy required for this will be more than as compared to the above two.And this happens due to the size of boron which is very small and the net addition of the all three ionization energies will be very high so the assertion is correct and the reason is the correct explanation for the assertion.

So (A)option is correct.

Note:Boron is only the compound in this group to form the covalent bond. Rather than it all the other forms the ionic bonds of 13th group.Boron constitutes about 0.001 percent by weight of earths crust.It is the metalloid, has 6 numbers of neutrons, 5 protons, generally found in solid state melting point is $2300^oC$ and boiling point of $2550^oC$.