\(a,10^{30}=\left(10^3\right)^{10}=1000^{10}\)
\(2^{100}=\left(2^{10}\right)^{10}=1024^{10}\)
\(1000^{10}< 1024^{10}\Rightarrow10^{30}< 2^{100}\)
\(b,2^{91}=\left(2^{13}\right)^7=8192^7\)
\(5^{35}=\left(5^5\right)^7=3124^7\)
\(8192^7>3124^7\)
\(\Rightarrow2^{91}>5^{35}\)
\(c,2^{1000}=\left(2^{10}\right)^{100}=1024^{100}\)
\(5^{400}=\left(5^4\right)^{100}=625^{100}\)
\(1024^{100}>625^{100}\)
\(\Rightarrow2^{1000}>5^{400}\)