A.

12.65

When equal volumes of acid and base are mixed then resulting solution become alkaline if the concentration of base is taken high.
Let normality  of the solution after mixing 0.1 M
NaOH and 0.01 M HCl is N.
therefore,   N₁V₁ - N₂V₂ = NV
or   0.1 x 1 - 0.01 x 1 = N x 2
Since normality, of NaOH, is more than that of HCl. Hence, the resulting solution is alkaline.
Or
[OH^-] = N = 0.09/2 = 0.045 N
Or pOH = - log (0.045) = 1.35
therefore, pH = 14-pOH
= 14 - 1.35 = 12.65

A.

(CH)₃B

According to Lewis concept " Acids are electron acceptor and bases are electron donor" Since, electron deficient compounds also have an ability to accept electrons, these are regarded as acids.
Trimethylborane (CH)₃B have incomplete octet thus, act as Lewis acid.

A.

HNO₂ and NaNO₂

A pair constituent with an HNO₂ and NaNO₂ because HNO₂ is weak acid and NaNO₂ is a salt of the weak acid (HNO₂) with a strong base (NaOH). Hence, it is an example of acidic buffer solution.

C.

Na₂CO₃

The highest pH refers to the basic solution containing OH^- ions. Therefore, the basic salt releasing OH^- ions on hydrolysis will give highest pH in water.
Only the salt of a strong base and weak acid would release OH^-ion on hydrolysis. Among the given salts, Na₂CO₃ corresponds to the basic salt as it is formed by the neutralisation of NaOH [strong base] and H₂CO₃ [weak acid].

C.

Every AB₅ molecules does, in fact, have square pyramid structure

Every AB₅ molecule does not, in fact, have square pyramidal structure but AB₅ molecules have trigonal bipyramidal structures due to sp³d hybridisation.