correct answer is: a) 3\large\frac{3}{4}

Explanation

Given fractions are \large\frac{3}{4},\frac{5}{8} and \large\frac{15}{16}
The formula we use to get the LCM of two or more fractions is:

\large\frac{The\ LCM\ of\ Numerator}{The\ HCF\ of\ Denominator}

Here, given numerators are 3,5 and 15.
\therefore The LCM of 3,5 and 15 is =15
And given denominators are 4,8 and 16
\therefore The HCF of 4,8 and 16 are =4
So that, the LCM of \large\frac{3}{4},\large\frac{5}{8} and \large\frac{15}{16} is =\large\frac{15}{4} or 3\large\frac{3}{4}
Ans: LCM 3\large\frac{3}{4}.

HCF = Highest Common Factor.
LCM = Least Common Multiple.