MySQL 8.0

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WITH yourTable AS (
    SELECT 1 AS section, 'A' AS sub_section, 'Foo' AS category UNION ALL
    SELECT 1, 'A', 'Bar' UNION ALL
    SELECT 1, 'A', 'Foo' UNION ALL
    SELECT 1, 'B', '123' UNION ALL
    SELECT 2, 'A', 'Bar' UNION ALL
    SELECT 2, 'A', 'Bar' UNION ALL
    SELECT 2, 'A', 'Bar' UNION ALL
    SELECT 2, 'A', 'Foo' UNION ALL
    SELECT 3, 'A', '123' UNION ALL
    SELECT 3, 'A', '123' UNION ALL
    SELECT 3, 'B', 'Bar' UNION ALL
    SELECT 4, 'A', 'Foo'
)

SELECT s1.section, s1.sub_section, s1.category
FROM
(
    SELECT section, sub_section, category, COUNT(*) AS cnt
    FROM yourTable
    GROUP BY section, sub_section, category
) s1
INNER JOIN
(
    SELECT section, sub_section, MAX(cnt) AS max_cnt
    FROM
    (
        SELECT section, sub_section, category, COUNT(*) AS cnt
        FROM yourTable
        GROUP BY section, sub_section, category
    ) t
    GROUP BY section, sub_section
) s2
    ON s2.section = s1.section AND
       s2.sub_section = s1.sub_section AND
       s1.cnt = s2.max_cnt
ORDER BY s1.section, s1.sub_section, s1.category

section	sub_section	category
1	A	Foo
1	B	123
2	A	Bar
3	A	123
3	B	Bar
4	A	Foo