$$\begin{gathered} 26. {\text{Earth moves in circular orbit around the sun once every year with an orbital radius of 1.5 × 10}}^{11}\text{m. } \\ {\text{what is the acceleration of Earth ( or any object on the surface of the Earth) towards the centre of the sun? How does this acceleration compare with g=9.8 ms}}^{-2}\text{?} \end{gathered}$$

Here Actually We need to find centripetal acceleration of earth when it goes around the sun.
r = 1.5 x 10¹¹ m
The angular velocity of Earth, 

ω = 2π / T
ω = (2x3.14 )/(365.25x24x3600) rad/s
Now Centripetal acceleration is given by 
a = rω²
= 1.5 x 10¹¹ [(2x3.14 )/(365.25x24x3600) ]² m/s²

PS - I know the calculation is little bit complex but we need to do it.