Both statements are individually true, but they concern entirely unrelated matters — one is about voltage conventions, the other about a numeric code — so the answer is option 2.

The assertion — positive logic. In positive logic the more positive voltage represents 1 and the less positive represents 0. In 5 V TTL, 0 is nominally 0 V and 1 is nominally 5 V. The convention is arbitrary but must be declared, because negative logic — the more positive level standing for 0 — is equally workable, and the same physical gate performs a different logical function under the two readings. A gate that is AND in positive logic is OR in negative logic, and NAND becomes NOR, by De Morgan's theorem:

\(\overline{A\cdot B}=\overline{A}+\overline{B}\)

This is why data sheets specify the convention rather than leaving it to be assumed.

The reason — excess-3 is self-complementary. Also true. Excess-3 encodes a decimal digit as its BCD value plus 3:

In each row the second code is the bitwise complement of the first. So the 9's complement of a digit — the operation needed for subtraction by complement addition — is obtained simply by inverting every bit, which a row of inverters does in no time at all. The offset of 3 is what centres the code so this works, and it also guarantees that no digit is encoded as 0000, so an all-zero word can be distinguished from the digit zero.

The two statements share nothing. A is a statement about voltage levels at the electrical interface; R is a statement about the arithmetic properties of a decimal encoding. Excess-3's self-complementing property holds whatever voltages represent the bits, and the positive-logic convention applies to every code equally. R is true, but it explains nothing about A.

Hence, both are correct, but (R) is not the explanation of (A).