In ncert exemplar the answer is given secondary xylem!!
don't know how!

Secondary Phloem:
Fibres are elongated, thick-walled cells that provide mechanical support to the plant. They are commonly found in various plant tissues, including the secondary phloem, secondary xylem, primary phloem, and leaves. However, fibres are likely to be absent in the secondary phloem.

Secondary Xylem:
Although fibres are present in the secondary xylem, they are absent in the secondary phloem. The secondary phloem is a part of the bark and is responsible for conducting sugars and other organic nutrients from the leaves to the rest of the plant. Unlike the secondary xylem, which contains fibres for structural support, the secondary phloem primarily consists of sieve elements, companion cells, parenchyma cells, and in some cases, sclereids.

Primary Phloem:
Fibres are also absent in the primary phloem. The primary phloem is the outermost layer of cells in the vascular cylinder of roots and stems. It is responsible for the transport of sugars and other organic compounds produced by the leaves to the growing regions of the plant. However, the primary phloem mainly consists of sieve tube elements, companion cells, and parenchyma cells, with no fibres present.

Leaves:
Unlike the secondary phloem, both the primary and secondary xylem, leaves can contain fibres. Fibres in leaves provide structural support and contribute to the overall strength of the leaf. They are commonly found in the midrib and veins of leaves, strengthening these structures and preventing them from collapsing under their own weight.

Conclusion:
In summary, fibres are likely to be absent in the secondary phloem, primary phloem, and leaves. They are primarily present in the secondary xylem, where they provide mechanical support to the plant. Understanding the distribution and presence of fibres in different plant tissues helps in comprehending the structural adaptations of plants and their ability to withstand various environmental conditions.