$a+b*10^6+c*10^{12}$ has three terms

a where 1<a<2
$b*10^6$ where 1<b<2
$c*10^{12}$ where 1<c<2

Now, compared to $c*10^{12}$ a is negligible so we can ignore that.
Even $b*10^6$ is small as compared to $c*10^{12}$, but let's keep it for now and let's look at the answer choices

A. $b*10^9$ this will be way less than $b*10^{12}$ so will be way less than too $c*10^{12}$
=> NOT the closes to $a+b*10^6+c*10^{12}$

B. $c*10^{12}$
Let's keep this as this is close to $b*10^6+c*10^{12}$

C. $(a+b+c)*10^6$
Now, even when we approximate a and b to be equal to c then the maximum value of this can still be
3c*10^6 which is way less than $b*10^6+c*10^{12}$

D. $(b+c-a)*10^{18}$
Now this gets multiplied with $10^{18}$ which will become too big as compared to $b*10^6+c*10^{12}$

E. $(a+b)*10^{12}$[/quote]
Now, lets assume a=1.1, b=1.2 and c=1.9
a+b = 2.3
=> $(a+b)*10^{12}$ = $2.3*10^{12}$ will become too big as compared to $1.1*10^6+1.9*10^{12}$

So, answer will be B
Hope it helps!