SOURCE

const tmp = [{
    a: 1,
    b: [{c: '2'}, { d: '3'}]
}, {
    a: 2,
    b: [{c: '4'}, { d: '5'}]
}];

function getResult(tag, data) {
    let result = [];
    data.forEach(res => {
        const zzz = res.b.filter(item => item.c === tag);
        if(zzz.length) {
            result.push(zzz);  
        }
    })
    return result;
}

console.log(getResult('4', tmp));