The Boolean equation for the circuit is

$(a)\;Y=A+\bar {B} \\ (b)\;Y= \overline {A+B} \\ (c)\;Y= \bar {A} +B \\ (d)\;y= \bar {A} +\bar {B}$

Boolean equation is:
$Y= A+ \bar {B}$
As,
represents NOT gate. and,
represents OR gate
Hence a is the correct answer.
edited Jul 13, 2014