Data offset (4 bits): Specifies the size of the TCP header in 32-bit words. The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. This field gets its name from the fact that it is also the offset from the start of the TCP segment to the actual data.
https://tools.ietf.org/html/rfc793
Data Offset: 4 bits
The number of 32 bit words in the TCP Header. This indicates the data begins.
The TCP header (even one including options) is an integral number of 32 bits long.
According to wikipedia (and rfc793):
Data offset (4 bits): Specifies the size of the TCP header in 32-bit words. The minimum size header is 5 words and the maximum is 15 words thus giving the minimum size of 20 bytes and maximum of 60 bytes, allowing for up to 40 bytes of options in the header. This field gets its name from the fact that it is also the offset from the start of the TCP segment to the actual data.
https://tools.ietf.org/html/rfc793 Data Offset: 4 bits The number of 32 bit words in the TCP Header. This indicates the data begins. The TCP header (even one including options) is an integral number of 32 bits long.