The CONCATENATE() function combines the contents of two or more cells (text1, [text2,…]) into a single cell, exactly matching the requirement to “combine multiple cells into a single cell.”

A. CONCAT() accepts only two arguments, so it cannot combine an arbitrary number of cells.

B. MERGE() is not a Sheets function; merging cells is a formatting command, not a formula.

D. JOIN() can combine cell values, but it always requires a delimiter; the question asks for the general-purpose combine function, which is CONCATENATE().

1. Google Docs Editors Help – Function list

“CONCATENATE” article

Syntax & Usage section (support.google.com/docs/answer/3094147).

2. Google Docs Editors Help – Function list

“CONCAT” article

Syntax section (support.google.com/docs/answer/3093472).

3. Google Docs Editors Help – Function list

“JOIN” article

Syntax section (support.google.com/docs/answer/3094136).