A. Expressions are juxtaposed
B. Expressions are multiplied
C. Cross operation
D. None of the mentioned
I got this question in an online interview.
Origin of the question is Building Regular Expressions topic in division Regular Expressions and Languages of Automata Theory
Easy explanation: Dot operation or concatenation operation means that the two expressions are juxtaposed i.e. there are no intervening operators in between. In fact, UNIX regular expressions use the dot for an entirely different purpose: representing any ASCII character.