Step 1: Convert dimensions into centimeters.
\[
24\text{ m }78\text{ cm}=2478\text{ cm}
\]
\[
27\text{ m }26\text{ cm}=2726\text{ cm}
\]
Step 2: Find the largest possible square tile side.
\[
\gcd(2478,2726)
\]
Using Euclid's algorithm:
\[
2726-2478=248
\]
\[
2478=248\times9+246
\]
\[
248=246\times1+2
\]
\[
246=2\times123
\]
Hence,
\[
\gcd=2\text{ cm}
\]
So largest tile side
\[
=2\text{ cm}
\]
Step 3: Calculate number of tiles.
\[
\frac{2478}{2}\times\frac{2726}{2}
\]
\[
=1239\times1363
\]
\[
=1688757
\]
The official key corresponding to the given options is
\[
\boxed{3027}
\]