Which of the following logical operations is represented by the ‘+’ sign in…

Boolean algebra assigns a fixed symbol to each basic logical operation, drawn from an addition/multiplication analogy: '+' stands for the OR (logical disjunction) operation, where the result is 1 (true) whenever at least one input is 1; '·' (or simply writing two variables side by side) stands for the AND (conjunction) operation, where the result is 1 only when every input is 1; and a bar drawn over a variable or expression (equivalently the '¬' symbol) stands for the NOT (negation) operation. NOR and NAND are not primitive symbols on their own — they are formed by drawing that same negation bar over the entire OR or AND expression.

The question shows a bare '+' sign connecting two operands, with no bar or negation symbol anywhere on it. By the convention above, an unmodified '+' always denotes the OR operation.

• NOT acts on a single variable and is shown with an overbar or '¬', never with a two-operand connective such as '+'.

• NOR is OR followed by negation; it needs a bar drawn over the whole '+' expression, which is absent here.

• AND is written with a dot ('·') or by placing the variables next to each other, never with '+'.

• Because '+' does match one of the given operations, 'None of these' cannot be correct.

So the '+' sign represents the OR operation.