Bit stuffing refers to

Bit stuffing is a data-link-layer framing technique that guarantees transparency: it prevents a bit pattern that matches the reserved flag sequence (typically 01111110) from occurring by accident inside the user's own data.

Applying this to the frame content here, the sender scans the outgoing user data stream and, whenever it detects five consecutive 1 bits, inserts a single extra 0 bit immediately after them, regardless of what bit follows. This guarantees the stream can never accidentally contain six 1s in a row, so it can never be confused with the flag pattern. On the receiving side, the destuffing process removes that inserted 0 after every run of five 1s, exactly reversing the operation and recovering the original data. This is precisely the bit-stuffing definition: inserting a 0 into the user stream to keep it distinguishable from a flag.

Each other option changes either the wrong stream or the wrong unit of insertion:

• Targeting the flag stream (rather than the user data) would mean altering the one fixed, pre-agreed marker both ends rely on to detect frame boundaries — the flag must stay constant, so nothing is ever inserted into it.

• Appending a nibble (4 bits) to the flag sequence combines both problems: it changes the reserved marker and does so in a coarser, unconditional unit rather than the single conditional bit the technique actually adds.

• Appending a nibble to the user data stream applies the right target (the data) but the wrong unit and the wrong trigger: a fixed 4-bit block added regardless of context is not equivalent to a single bit inserted only after five consecutive 1s.