Inserting 4, 16 and 64 between 1 and 256 makes the resulting sequence a:

1. Each of these numbers is four times the one before it. 2. A constant multiplier is the signature of one kind of progression. 3. Inserting 4, 16, 64 between 1 and 256 makes the resulting sequences in G.P. 4. So the answer is a geometric progression. _Source: NCERT Class XI Mathematics, Ch 8 'Sequences and Series'_